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/clutter-main.c

@@ -2410,7 +2410,7 @@ clutter_do_event (ClutterEvent *event)
    * because we've "looked ahead" and know all motion events that
    * will occur before drawing the frame.
    */
-  _clutter_stage_queue_event (event->any.stage, event);
+  _clutter_stage_queue_event (event->any.stage, event, TRUE);
 }
 
 static void