Stop audio in iOS when backgrounded.

- Fixes a crash when a sound is still playing and Artboard is de-allocated. Easy to cause with long sounds as sound resources would get nuked and the engine would keep trying to decode from a dead buffer.
- Stops audio when going in background, resumes when coming to foreground.

The way I did the backgrounding is on the RiveRendererView as I couldn't find a better general spot to do this but maybe @mjtalbot has an idea.

This does not stop long sounds (like songs) when exiting a view, we can address that separately...

Diffs=
4a9947630 Stop audio in iOS when backgrounded. (#7055)

Co-authored-by: Luigi Rosso <luigi-rosso@users.noreply.github.com>

.rive_head

@@ -1 +1 @@
-cb2ea5b2d31eb6888a96cb25524392087bdf47c7
+4a9947630d3848ff70813cf5e2d8718adb9a1b90

.rive_renderer

@@ -1 +1 @@
-0eeb44647783f7ad0fc3607c9f7be76bce9fdea4
+ecf23d0e1921e3d8318e2c9a75f18911aa4daaf4

Source/Renderer/rive_renderer_view.mm

@@ -12,15 +12,36 @@
 #import "RivePrivateHeaders.h"
 #import <RenderContext.h>
 #import <RenderContextManager.h>
+// We manually need to provide this as our build-time config isn't shared with xcode.
+#define WITH_RIVE_AUDIO
+#include "rive/audio/audio_engine.hpp"
 
 @implementation RiveRendererView
 {
     RenderContext* _renderContext;
     rive::Renderer* _renderer;
 }
 
+- (void)didEnterBackground:(NSNotification*)notification
+{
+    rive::AudioEngine::RuntimeEngine()->stop();
+}
+
+- (void)didEnterForeground:(NSNotification*)notification
+{
+    rive::AudioEngine::RuntimeEngine()->start();
+}
+
 - (instancetype)initWithCoder:(NSCoder*)decoder
 {
+    [[NSNotificationCenter defaultCenter] addObserver:self
+                                             selector:@selector(didEnterBackground:)
+                                                 name:UIApplicationDidEnterBackgroundNotification
+                                               object:nil];
+    [[NSNotificationCenter defaultCenter] addObserver:self
+                                             selector:@selector(didEnterForeground:)
+                                                 name:UIApplicationWillEnterForegroundNotification
+                                               object:nil];
     self = [super initWithCoder:decoder];
 
     _renderContext = [[RenderContextManager shared] getDefaultContext];
@@ -36,6 +57,14 @@ - (instancetype)initWithCoder:(NSCoder*)decoder
 
 - (instancetype)initWithFrame:(CGRect)frameRect
 {
+    [[NSNotificationCenter defaultCenter] addObserver:self
+                                             selector:@selector(didEnterBackground:)
+                                                 name:UIApplicationDidEnterBackgroundNotification
+                                               object:nil];
+    [[NSNotificationCenter defaultCenter] addObserver:self
+                                             selector:@selector(didEnterForeground:)
+                                                 name:UIApplicationWillEnterForegroundNotification
+                                               object:nil];
     _renderContext = [[RenderContextManager shared] getDefaultContext];
     assert(_renderContext);