clutter: Make crossing events unstoppable

These events are not meant to be ever silenced away, every actor
that is meant to receive one should do so. Make it sure that those
events cannot be stopped, despite the event signal handlers return
values.

This opens the debate about whether crossing events should be
ClutterEvents, since they are more and more uncommon at being one,
maybe this notification mechanism should be taken away from the
event machinery, but that's something for future refactors.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2068>

clutter/clutter/clutter-actor.c

@@ -12388,6 +12388,12 @@ clutter_actor_event (ClutterActor       *actor,
  handled:
   g_object_unref (actor);
 
+  if (event->type == CLUTTER_ENTER || event->type == CLUTTER_LEAVE)
+    {
+      g_warn_if_fail (retval == CLUTTER_EVENT_PROPAGATE);
+      return CLUTTER_EVENT_PROPAGATE;
+    }
+
   return retval;
 }