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.cpp

@@ -30,6 +30,8 @@
 
 #include "rendering_server_globals.h"
 
+bool RenderingServerGlobals::threaded = false;
+
 RendererStorage *RenderingServerGlobals::storage = nullptr;
 RendererCanvasRender *RenderingServerGlobals::canvas_render = nullptr;
 RendererCompositor *RenderingServerGlobals::rasterizer = nullptr;