Implement async shader compilation plus caching for GL ES 3

Async. compilation via ubershader is currently available in the scene and particles shaders only.

Bonus:
- Use `#if defined()` syntax for not true conditionals, so they don't unnecessarily take a bit in the version flagset.
- Remove unused `ENABLE_CLIP_ALPHA` from scene shader.
- Remove unused `PARTICLES_COPY` from the particles shader.
- Remove unused uniform related code.
- Shader language/compiler: use ordered hash maps for deterministic code generation (needed for caching).

editor/editor_export.cpp

@@ -268,6 +268,10 @@ void EditorExportPlatform::gen_debug_flags(Vector<String> &r_flags, int p_flags)
 	if (p_flags & DEBUG_FLAG_VIEW_NAVIGATION) {
 		r_flags.push_back("--debug-navigation");
 	}
+
+	if (p_flags & DEBUG_FLAG_SHADER_FALLBACKS) {
+		r_flags.push_back("--debug-shader-fallbacks");
+	}
 }
 
 Error EditorExportPlatform::_save_pack_file(void *p_userdata, const String &p_path, const Vector<uint8_t> &p_data, int p_file, int p_total) {