backend-x11: Take the touch grab on the stage window

So we don't get assert fails when touching the screen on mutter nested.

src/core/display.c

@@ -822,6 +822,8 @@ meta_display_open (void)
 
   enable_compositor (display);
 
+  meta_backend_compositor_created (meta_get_backend ());
+
   meta_screen_create_guard_window (screen);
 
   /* Set up touch support */