Add AudioStreamPolyphonic to make it easier to play polyphonic sound …

…from code

* This new audio stream allows to play multiple sounds and control them over time from code.
* It greatly simplifies tasks such as generative music (music generated from code) or audio.

This new type of stream was added with the goal of fixing audio blending in AnimationPlayer and AnimationTree, but can be used by others for their regular audio needs.

Does not fix anything currently, but should help implement godotengine#69758 properly.

Some demo code of how to use this:

```GDScript

var player = $SomeNode as AudioStreamPlayer
player.stream = AudioStreamPolyphonic.new()
var playback = player.get_stream_playback() as AudioStreamPlaybackPolyphonic
var id = playback.play_stream(preload("res://Clip1.ogg"))
await get_tree().create_timer(1).timeout
playback.set_stream_volume(id,-12) # Set volume to half after one second
await get_tree().create_timer(2).timeout
var id2 = playback.play_stream(preload("res://Clip2.ogg")) # 2 seconds later, start another clip
await get_tree().create_timer(1).timeout
playback.stop_stream(id) # 1 second later, kill the first clip
playback.set_stream_pitch_scale(id2,1.5) # Make the second clip go 50% faster

```

doc/classes/AudioStreamPlaybackPolyphonic.xml

@@ -0,0 +1,61 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<class name="AudioStreamPlaybackPolyphonic" inherits="AudioStreamPlayback" version="4.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
+	<brief_description>
+		Playback instance for [AudioStreamPolyphonic].
+	</brief_description>
+	<description>
+		Playback instance for [AudioStreamPolyphonic]. After setting the [code]stream[/code] property of [AudioStreamPlayer], [AudioStreamPlayer2D], or [AudioStreamPlayer3D], the playback instance can be obtained by calling [method AudioStreamPlayer.get_stream_playback], [method AudioStreamPlayer2D.get_stream_playback] or [method AudioStreamPlayer3D.get_stream_playback] methods.
+	</description>
+	<tutorials>
+	</tutorials>
+	<methods>
+		<method name="is_stream_playing" qualifiers="const">
+			<return type="bool" />
+			<param index="0" name="stream" type="int" />
+			<description>
+				Return true whether the stream associated with an integer ID is still playing. Check [method play_stream] for information on when this ID becomes invalid.
+			</description>
+		</method>
+		<method name="play_stream">
+			<return type="int" />
+			<param index="0" name="stream" type="AudioStream" />
+			<param index="1" name="from_offset" type="float" default="0" />
+			<param index="2" name="volume_db" type="float" default="0" />
+			<param index="3" name="pitch_scale" type="float" default="1.0" />
+			<description>
+				Play an [AudioStream] at a given offset, volume and pitch scale. Playback starts immediately.
+				The return value is an unique integer ID that is associated to this playback stream and which can be used to controll it.
+				This ID becomes invalid when the stream ends (if it does not loop), when the [AudioStreamPlaybackPolyphonic] is stopped, or when [method stop_stream] is called.
+				This function returns [constant INVALID_ID] if the amount of streams currently playing equals [member AudioStreamPolyphonic.polyphony]. If you need a higher amount of maximum polyphony, raise this value.
+			</description>
+		</method>
+		<method name="set_stream_pitch_scale">
+			<return type="void" />
+			<param index="0" name="stream" type="int" />
+			<param index="1" name="pitch_scale" type="float" />
+			<description>
+				Change the stream pitch scale. The [param stream] argument is an integer ID returned by [method play_stream].
+			</description>
+		</method>
+		<method name="set_stream_volume">
+			<return type="void" />
+			<param index="0" name="stream" type="int" />
+			<param index="1" name="volume_db" type="float" />
+			<description>
+				Change the stream volume (in db). The [param stream] argument is an integer ID returned by [method play_stream].
+			</description>
+		</method>
+		<method name="stop_stream">
+			<return type="void" />
+			<param index="0" name="stream" type="int" />
+			<description>
+				Stop a stream. The [param stream] argument is an integer ID returned by [method play_stream], which becomes invalid after calling this function.
+			</description>
+		</method>
+	</methods>
+	<constants>
+		<constant name="INVALID_ID" value="-1">
+			Returned by [method play_stream] in case it could not allocate a stream for playback.
+		</constant>
+	</constants>
+</class>

doc/classes/AudioStreamPolyphonic.xml

@@ -0,0 +1,17 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<class name="AudioStreamPolyphonic" inherits="AudioStream" version="4.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
+	<brief_description>
+		AudioStream that lets the user play custom streams at any time from code, simultaneously using a single player.
+	</brief_description>
+	<description>
+		AudioStream that lets the user play custom streams at any time from code, simultaneously using a single player.
+		Playback control is done via the [AudioStreamPlaybackPolyphonic] instance set inside the player, which can be obtained via [method AudioStreamPlayer.get_stream_playback], [method AudioStreamPlayer2D.get_stream_playback] or [method AudioStreamPlayer3D.get_stream_playback] methods. Obtaining the playback instance is only valid after the [code]stream[/code] property is set as an [AudioStreamPolyphonic] in those players.
+	</description>
+	<tutorials>
+	</tutorials>
+	<members>
+		<member name="polyphony" type="int" setter="set_polyphony" getter="get_polyphony" default="32">
+			Maximum amount of simultaneous streams that can be played.
+		</member>
+	</members>
+</class>

scene/register_scene_types.cpp

@@ -140,6 +140,7 @@
 #include "scene/main/viewport.h"
 #include "scene/main/window.h"
 #include "scene/resources/animation_library.h"
+#include "scene/resources/audio_stream_polyphonic.h"
 #include "scene/resources/audio_stream_wav.h"
 #include "scene/resources/bit_map.h"
 #include "scene/resources/bone_map.h"
@@ -907,6 +908,8 @@ void register_scene_types() {
 #endif
 	GDREGISTER_ABSTRACT_CLASS(VideoStream);
 	GDREGISTER_CLASS(AudioStreamWAV);
+	GDREGISTER_CLASS(AudioStreamPolyphonic);
+	GDREGISTER_ABSTRACT_CLASS(AudioStreamPlaybackPolyphonic);
 
 	OS::get_singleton()->yield(); // may take time to init
 

scene/resources/audio_stream_polyphonic.cpp

@@ -0,0 +1,286 @@
+/**************************************************************************/
+/*  audio_stream_polyphonic.cpp                                           */
+/**************************************************************************/
+/*                         This file is part of:                          */
+/*                             GODOT ENGINE                               */
+/*                        https://godotengine.org                         */
+/**************************************************************************/
+/* Copyright (c) 2014-present Godot Engine contributors (see AUTHORS.md). */
+/* Copyright (c) 2007-2014 Juan Linietsky, Ariel Manzur.                  */
+/*                                                                        */
+/* Permission is hereby granted, free of charge, to any person obtaining  */
+/* a copy of this software and associated documentation files (the        */
+/* "Software"), to deal in the Software without restriction, including    */
+/* without limitation the rights to use, copy, modify, merge, publish,    */
+/* distribute, sublicense, and/or sell copies of the Software, and to     */
+/* permit persons to whom the Software is furnished to do so, subject to  */
+/* the following conditions:                                              */
+/*                                                                        */
+/* The above copyright notice and this permission notice shall be         */
+/* included in all copies or substantial portions of the Software.        */
+/*                                                                        */
+/* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,        */
+/* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF     */
+/* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. */
+/* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY   */
+/* CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,   */
+/* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE      */
+/* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                 */
+/**************************************************************************/
+
+#include "audio_stream_polyphonic.h"
+#include "scene/main/scene_tree.h"
+
+Ref<AudioStreamPlayback> AudioStreamPolyphonic::instantiate_playback() {
+	Ref<AudioStreamPlaybackPolyphonic> playback;
+	playback.instantiate();
+	playback->streams.resize(polyphony);
+	return playback;
+}
+
+String AudioStreamPolyphonic::get_stream_name() const {
+	return "AudioStreamPolyphonic";
+}
+
+bool AudioStreamPolyphonic::is_monophonic() const {
+	return true; // This avoids stream players to instantiate more than one of these.
+}
+
+void AudioStreamPolyphonic::set_polyphony(int p_voices) {
+	ERR_FAIL_COND(p_voices < 0 || p_voices > 128);
+	polyphony = p_voices;
+}
+int AudioStreamPolyphonic::get_polyphony() const {
+	return polyphony;
+}
+
+void AudioStreamPolyphonic::_bind_methods() {
+	ClassDB::bind_method(D_METHOD("set_polyphony", "voices"), &AudioStreamPolyphonic::set_polyphony);
+	ClassDB::bind_method(D_METHOD("get_polyphony"), &AudioStreamPolyphonic::get_polyphony);
+
+	ADD_PROPERTY(PropertyInfo(Variant::INT, "polyphony", PROPERTY_HINT_RANGE, "1,128,1"), "set_polyphony", "get_polyphony");
+}
+
+AudioStreamPolyphonic::AudioStreamPolyphonic() {
+}
+
+////////////////////////
+
+void AudioStreamPlaybackPolyphonic::start(double p_from_pos) {
+	if (active) {
+		stop();
+	}
+
+	active = true;
+}
+
+void AudioStreamPlaybackPolyphonic::stop() {
+	if (!active) {
+		return;
+	}
+
+	bool locked = false;
+	for (Stream &s : streams) {
+		if (s.active.is_set()) {
+			// Need locking because something may still be mixing.
+			locked = true;
+			AudioServer::get_singleton()->lock();
+		}
+		s.active.clear();
+		s.finalizing.clear();
+		s.finish_request.clear();
+		s.stream_playback.unref();
+		s.stream.unref();
+	}
+	if (locked) {
+		AudioServer::get_singleton()->unlock();
+	}
+
+	active = false;
+}
+
+bool AudioStreamPlaybackPolyphonic::is_playing() const {
+	return active;
+}
+
+int AudioStreamPlaybackPolyphonic::get_loop_count() const {
+	return 0;
+}
+
+double AudioStreamPlaybackPolyphonic::get_playback_position() const {
+	return 0;
+}
+void AudioStreamPlaybackPolyphonic::seek(double p_time) {
+	// Ignored.
+}
+
+void AudioStreamPlaybackPolyphonic::tag_used_streams() {
+	for (Stream &s : streams) {
+		if (s.active.is_set()) {
+			s.stream_playback->tag_used_streams();
+		}
+	}
+}
+
+int AudioStreamPlaybackPolyphonic::mix(AudioFrame *p_buffer, float p_rate_scale, int p_frames) {
+	if (!active) {
+		return 0;
+	}
+
+	// Pre-clear buffer.
+	for (int i = 0; i < p_frames; i++) {
+		p_buffer[i] = AudioFrame(0, 0);
+	}
+
+	for (Stream &s : streams) {
+		if (!s.active.is_set()) {
+			continue;
+		}
+
+		float volume_db = s.volume_db; // Copy because it can be overriden at any time.
+		float next_volume = Math::db_to_linear(volume_db);
+		s.prev_volume_db = volume_db;
+
+		if (s.finish_request.is_set()) {
+			if (s.pending_play.is_set()) {
+				// Did not get the chance to play, was finalized too soon.
+				s.active.clear();
+				s.finalizing.set();
+				continue;
+			}
+			next_volume = 0;
+		}
+
+		if (s.pending_play.is_set()) {
+			s.stream_playback->start(s.play_offset);
+			s.pending_play.clear();
+		}
+		float prev_volume = Math::db_to_linear(s.prev_volume_db);
+
+		float volume_inc = (next_volume - prev_volume) / float(p_frames);
+
+		int todo = p_frames;
+		int offset = 0;
+		float volume = prev_volume;
+
+		while (todo) {
+			int to_mix = MIN(todo, int(INTERNAL_BUFFER_LEN));
+			int mixed = s.stream_playback->mix(internal_buffer, s.pitch_scale, to_mix);
+
+			for (int i = 0; i < to_mix; i++) {
+				p_buffer[offset + i] += internal_buffer[i] * volume;
+				volume += volume_inc;
+			}
+
+			if (mixed < to_mix) {
+				// Stream is done.
+				s.active.clear();
+				s.finalizing.set();
+				break;
+			}
+
+			todo -= to_mix;
+			offset += to_mix;
+		}
+
+		if (s.finish_request.is_set()) {
+			s.active.clear();
+			s.finalizing.set();
+		}
+	}
+
+	return p_frames;
+}
+
+void AudioStreamPlaybackPolyphonic::_check_finalized_streams() {
+	if (!active) {
+		return;
+	}
+
+	for (Stream &s : streams) {
+		if (!s.active.is_set() && s.finalizing.is_set()) {
+			s.stream_playback.unref();
+			s.stream.unref();
+			s.finalizing.clear();
+			s.finish_request.clear();
+		}
+	}
+}
+
+AudioStreamPlaybackPolyphonic::ID AudioStreamPlaybackPolyphonic::play_stream(const Ref<AudioStream> &p_stream, float p_from_offset, float p_volume_db, float p_pitch_scale) {
+	ERR_FAIL_COND_V(p_stream.is_null(), INVALID_ID);
+	for (uint32_t i = 0; i < streams.size(); i++) {
+		if (!streams[i].active.is_set() && !streams[i].finish_request.is_set() && !streams[i].finalizing.is_set()) {
+			// Can use this stream, as it's not active.
+			streams[i].stream = p_stream;
+			streams[i].stream_playback = streams[i].stream->instantiate_playback();
+			streams[i].play_offset = p_from_offset;
+			streams[i].volume_db = p_volume_db;
+			streams[i].prev_volume_db = p_volume_db;
+			streams[i].pitch_scale = p_pitch_scale;
+			streams[i].id = id_counter++;
+			streams[i].pending_play.set();
+			streams[i].active.set();
+			return (ID(i) << INDEX_SHIFT) | ID(streams[i].id);
+		}
+	}
+
+	return INVALID_ID;
+}
+
+AudioStreamPlaybackPolyphonic::Stream *AudioStreamPlaybackPolyphonic::_find_stream(int64_t p_id) {
+	uint32_t index = p_id >> INDEX_SHIFT;
+	if (index >= streams.size()) {
+		return nullptr;
+	}
+	if (!streams[index].active.is_set()) {
+		return nullptr; // Not active, no longer exists.
+	}
+	int64_t id = p_id & ID_MASK;
+	if (streams[index].id != id) {
+		return nullptr;
+	}
+	return &streams[index];
+}
+
+void AudioStreamPlaybackPolyphonic::set_stream_volume(ID p_stream_id, float p_volume_db) {
+	Stream *s = _find_stream(p_stream_id);
+	if (!s) {
+		return;
+	}
+	s->volume_db = p_volume_db;
+}
+
+void AudioStreamPlaybackPolyphonic::set_stream_pitch_scale(ID p_stream_id, float p_pitch_scale) {
+	Stream *s = _find_stream(p_stream_id);
+	if (!s) {
+		return;
+	}
+	s->pitch_scale = p_pitch_scale;
+}
+
+bool AudioStreamPlaybackPolyphonic::is_stream_playing(ID p_stream_id) const {
+	return const_cast<AudioStreamPlaybackPolyphonic *>(this)->_find_stream(p_stream_id) != nullptr;
+}
+
+void AudioStreamPlaybackPolyphonic::stop_stream(ID p_stream_id) {
+	Stream *s = _find_stream(p_stream_id);
+	if (!s) {
+		return;
+	}
+	s->finish_request.set();
+}
+
+void AudioStreamPlaybackPolyphonic::_bind_methods() {
+	ClassDB::bind_method(D_METHOD("play_stream", "stream", "from_offset", "volume_db", "pitch_scale"), &AudioStreamPlaybackPolyphonic::play_stream, DEFVAL(0), DEFVAL(0), DEFVAL(1.0));
+	ClassDB::bind_method(D_METHOD("set_stream_volume", "stream", "volume_db"), &AudioStreamPlaybackPolyphonic::set_stream_volume);
+	ClassDB::bind_method(D_METHOD("set_stream_pitch_scale", "stream", "pitch_scale"), &AudioStreamPlaybackPolyphonic::set_stream_pitch_scale);
+	ClassDB::bind_method(D_METHOD("is_stream_playing", "stream"), &AudioStreamPlaybackPolyphonic::is_stream_playing);
+	ClassDB::bind_method(D_METHOD("stop_stream", "stream"), &AudioStreamPlaybackPolyphonic::stop_stream);
+
+	BIND_CONSTANT(INVALID_ID);
+}
+
+AudioStreamPlaybackPolyphonic::AudioStreamPlaybackPolyphonic() {
+	SceneTree::get_singleton()->connect(SNAME("process_frame"), callable_mp(this, &AudioStreamPlaybackPolyphonic::_check_finalized_streams));
+}