Use the Compatibility renderer

The game is a 2D interface without 3D, shaders or lights, so it does
not need Forward+. With Compatibility it uses about 25% less CPU and
30 MB less memory while running, with no visible difference.

project.godot

@@ -17,7 +17,7 @@ compatibility/default_parent_skeleton_in_mesh_instance_3d=true
 config/name="MusicPlayer"
 config/version="1.9.0"
 run/main_scene="res://MainWindow.tscn"
-config/features=PackedStringArray("4.7", "Forward Plus")
+config/features=PackedStringArray("4.7", "GL Compatibility")
 config/icon="res://icon.svg"
 
 [autoload]
@@ -61,6 +61,9 @@ ui_select={
 
 [rendering]
 
+renderer/rendering_method="gl_compatibility"
+renderer/rendering_method.mobile="gl_compatibility"
+
 textures/vram_compression/import_s3tc_bptc=true
 textures/vram_compression/import_etc2_astc=true
 textures/default_filters/use_nearest_mipmap_filter=true