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

doc/classes/EditorInterface.xml

@@ -57,6 +57,12 @@
 				[b]Note:[/b] This returns the main editor control containing the whole editor, not the 2D or 3D viewports specifically.
 			</description>
 		</method>
+		<method name="get_editor_paths">
+			<return type="EditorPaths">
+			</return>
+			<description>
+			</description>
+		</method>
 		<method name="get_editor_scale" qualifiers="const">
 			<return type="float">
 			</return>