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/win32/clutter-event-win32.c

@@ -294,8 +294,7 @@ clutter_event_dispatch (GSource     *source,
   if ((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);
     }
 
   _clutter_threads_release_lock ();