Refactor VisibilityNotifier

* Works from RenderinServer
* Accurately tells when on or off-scren, its no longer approximate.
* VisibilityEnabler also simplified to use the process mode instead.

servers/rendering/rendering_server_globals.h

@@ -41,6 +41,8 @@ class RendererScene;
 
 class RenderingServerGlobals {
 public:
+	static bool threaded;
+
 	static RendererStorage *storage;
 	static RendererCanvasRender *canvas_render;
 	static RendererCompositor *rasterizer;