Avoid needless event copies when queueing from a backend to a stage

All backends follow the same pattern of queueing events first in
ClutterMainContext, then copying them to a ClutterStage queue and
immediately free them. Instead, we can just pass ownership of events
directly to ClutterStage thus avoiding the allocation and copy in
between.

https://bugzilla.gnome.org/show_bug.cgi?id=711857

clutter/gdk/clutter-event-gdk.c

@@ -309,8 +309,7 @@ clutter_gdk_handle_event (GdkEvent *gdk_event)
       while (spin > 0 && (event = clutter_event_get ()))
 	{
 	  /* forward the event into clutter for emission etc. */
-	  clutter_do_event (event);
-	  clutter_event_free (event);
+          _clutter_stage_queue_event (event->any.stage, event, FALSE);
 	  --spin;
 	}