actor: warn if allocating actor not descending from stage

This adds a verbose warning that will be displayed if
clutter_actor_allocate is passed an actor that isn't a descendent of a
ClutterStage. Layouting should always bubble up from a stage so this
condition is likely to indicate a buggy container that allocating a
child that it has already unparented.

clutter/clutter-actor.c

@@ -5282,6 +5282,13 @@ clutter_actor_allocate (ClutterActor           *self,
   gboolean child_moved;
 
   g_return_if_fail (CLUTTER_IS_ACTOR (self));
+  if (G_UNLIKELY (_clutter_actor_get_stage_internal (self) == NULL))
+    {
+      g_warning ("Spurious clutter_actor_allocate called for actor %p/%s "
+                 "which isn't a descendent of the stage!\n",
+                 self, G_OBJECT_TYPE_NAME (self));
+      return;
+    }
 
   priv = self->priv;