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/display_server.cpp

@@ -208,6 +208,10 @@ void DisplayServer::window_set_mouse_passthrough(const Vector<Vector2> &p_region
 	ERR_FAIL_MSG("Mouse passthrough not supported by this display server.");
 }
 
+void DisplayServer::gl_window_make_current(DisplayServer::WindowID p_window_id) {
+	// noop except in gles
+}
+
 void DisplayServer::window_set_ime_active(const bool p_active, WindowID p_window) {
 	WARN_PRINT("IME not supported by this display server.");
 }