Add g_warning to not yet supported code

gdk/gdkwindow.c

@@ -1081,6 +1081,8 @@ gdk_window_set_has_native (GdkWindow *window, gboolean has_native)
 	  GDK_WINDOW_TYPE (private->parent) == GDK_WINDOW_ROOT)
 	return; /* toplevel, must be native */
 
+      g_warning ("Tried to turn native window to client side window, this is not supported yet.");
+
       /* TODO: remove native */
     }
 }