clutter: Demote the new clutter_stage_grab() precondition check

There do indeed seem to be places in our own code that trigger grabs on
actors before they are realized. It was not the intention to change the
practical preconditions for GNOME 43, so make it an even lower minimum
that every caller ought to match: That the actor is attached to the stage.

Further constraining of these preconditions will have to wait until
branching for new development.

Fixes: 9c79c7234 (clutter: Only allow grabs to be created on realized actors)
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2670>

clutter/clutter/clutter-stage.c

@@ -3790,7 +3790,9 @@ clutter_stage_grab (ClutterStage *stage,
 
   g_return_val_if_fail (CLUTTER_IS_STAGE (stage), NULL);
   g_return_val_if_fail (CLUTTER_IS_ACTOR (actor), NULL);
-  g_return_val_if_fail (clutter_actor_is_realized (actor), NULL);
+  g_return_val_if_fail (stage ==
+                        (ClutterStage *) _clutter_actor_get_stage_internal (actor),
+                        NULL);
 
   priv = stage->priv;