clutter/actor: Set initial allocation to uninitialized ClutterActorBox

We currently initialize the ClutterActorBox of the actors allocation to
zero, but there's a difference between a valid zero-allocation and an
actor having never been allocated. Currently it's impossible for us to
detect the latter case in a reliable way and we use the needs_allocation
flag for this, which may also be set in other situations.

So initialize the allocation of actors to the newly added UNINITIALIZED
ClutterActorBox, which will make it easier to detect whether an actor
already got its initial allocation.

This also fixes another issue right now: Actors which get allocated a
(valid) zero allocation, will now notify the "allocation" property in
this case.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1290

clutter/clutter/clutter-actor.c

@@ -8694,6 +8694,8 @@ clutter_actor_init (ClutterActor *self)
 
   self->priv = priv = clutter_actor_get_instance_private (self);
 
+  priv->allocation = (ClutterActorBox) CLUTTER_ACTOR_BOX_UNINITIALIZED;
+
   priv->opacity = 0xff;
   priv->show_on_set_parent = TRUE;
   priv->resource_scale = -1.0f;