[TextServer / Font] Do not duplicate images to prevent unnecessary embedding.

[bruvzg]

Fixes #82254

Note: this only works if images were imported and loaded using load/preload, in other cases images are still embedded (there's no other way).

Images for the imported BMFonts are embedded as well, since it can be packed and require conversion before used (we probably should add ignore import option, and auto set it for the BMFont images).

e.g.,:

@tool
extends FontFile
class_name MyFont

const FONT_SIZE = Vector2i(16, 16)

func _init():
	clear_cache()
	
	var image = load("res://SuperWhite.png")
	set_texture_image(0, FONT_SIZE, 0, image)

[gd_resource type="FontFile" script_class="MyFont" load_steps=3 format=3 uid="uid://df8eka62scrfs"]

[ext_resource type="Image" uid="uid://d1bu5rv3qc5oe" path="res://SuperWhite.png" id="1_q7u5v"]
[ext_resource type="Script" path="res://my_font.gd" id="2_dlquq"]

[resource]
cache/0/16/16/textures/0/offsets = PackedInt32Array()
cache/0/16/16/textures/0/image = ExtResource("1_q7u5v")
cache/0/16/0/ascent = 0.0
cache/0/16/0/descent = 0.0
cache/0/16/0/underline_position = 0.0
cache/0/16/0/underline_thickness = 0.0
cache/0/16/0/scale = 1.0
cache/0/16/0/kerning_overrides/16/16 = Vector2(0, 0)
cache/0/16/0/kerning_overrides/16/0 = Vector2(0, 0)
script = ExtResource("2_dlquq")

[akien-mga]

Thanks!