Implement shader caching

* Shader compilation is now cached. Subsequent loads take less than a millisecond.
* Improved game, editor and project manager startup time.
* Editor uses .godot/shader_cache to store shaders.
* Game uses user://shader_cache
* Project manager uses $config_dir/shader_cache
* Options to tweak shader caching in project settings.
* Editor path configuration moved from EditorSettings to new class, EditorPaths, so it can be available early on (before shaders are compiled).
* Reworked ShaderCompilerRD to ensure deterministic shader code creation (else shader may change and cache will be invalidated).
* Added shader compression with SMOLV: https://github.com/aras-p/smol-v

editor/editor_node.h

@@ -798,6 +798,7 @@ public:
 
 	Error export_preset(const String &p_preset, const String &p_path, bool p_debug, bool p_pack_only);
 
+	static void register_editor_paths(bool p_for_project_manager);
 	static void register_editor_types();
 	static void unregister_editor_types();