Enable asynchronous shader compilation and caching

This mitigates shader compilation stutter, especially after an
initial playthrough thanks to caching.

This requires Godot 3.5 or later to have an effect.

project.godot

@@ -221,5 +221,6 @@ common/physics_interpolation=true
 vram_compression/import_etc2=false
 quality/shadow_atlas/size=2048
 quality/shading/use_physical_light_attenuation=true
+gles3/shaders/shader_compilation_mode=2
 quality/filters/msaa=2
 environment/default_environment="res://default_env.tres"