We might pick an actor that needs relayout. I've seen this happen inside
hiding / unmapping in particular. In this case, calculate_clear_area ()
will call clutter_actor_get_abs_allocation_vertices () which in turn
will force a relayout. However, this is not what we want, because:
1. We don't want to run layout during picking.
2. If the actor needs an allocation, then the pick stack could not have
used an up-to-date allocation, because it is not computed. Therefore
this clear area would use a potentially completely different
allocation than the one stored in the pick stack.
Thankfully, clear area seems to be used as a cache/optimization, so
let's just avoid computing it if the actor is not allocated.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3425>