Add GLES2 2D renderer + Linux display manager

First implementation with Linux display manager.

- Add single-threaded mode for EditorResourcePreview (needed for OpenGL).

Co-authored-by: clayjohn <claynjohn@gmail.com>
Co-authored-by: Fabio Alessandrelli <fabio.alessandrelli@gmail.com>

servers/rendering/renderer_scene_cull.cpp

@@ -3855,6 +3855,10 @@ void RendererSceneCull::update() {
 }
 
 bool RendererSceneCull::free(RID p_rid) {
+	if (p_rid.is_null()) {
+		return true;
+	}
+
 	if (scene_render->free(p_rid)) {
 		return true;
 	}