Only reset the relayout_pending flag when actually doing a relayout

If an actor is (unfortunately) queuing a relayout in relayout, you would
end up with (ClutterActor*)stage->needs_allocation set to TRUE and
stage->relayout_pending set to TRUE. But if then in the same cycle, an
actor calls clutter_actor_get_allocation_box, that will trigger another
(recursive) _clutter_stage_maybe_relayout, which will wrongly reset the
relayout pending to FALSE, while not actually performing a new relayout
because of the re-entrancy protection.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2503

clutter/clutter-stage.c

@@ -761,11 +761,12 @@ _clutter_stage_maybe_relayout (ClutterActor *actor)
 
   if (!priv->relayout_pending)
     return;
-  priv->relayout_pending = FALSE;
 
   /* avoid reentrancy */
   if (!CLUTTER_ACTOR_IN_RELAYOUT (stage))
     {
+      priv->relayout_pending = FALSE;
+
       CLUTTER_TIMER_START (_clutter_uprof_context, relayout_timer);
       CLUTTER_NOTE (ACTOR, "Recomputing layout");