clutter_actor_paint() implements a clear preference for custom clips
over clip_to_allocation: If a custom clip is set, clip_to_allocation is
ignored.
Since the paint volume reflects what Clutter is going to paint, we
should handle it the same when putting together our paint volume: So
first handle custom clips, and if one is set, use that. Then handle
clip_to_allocation, and if that's set, use that. And finally, if both
aren't set, union our allocation with the children paint volumes to get
the building volume.
clutter_actor_paint() also doesn't check whether the custom clip is
empty: If that's the case, it will simply not paint anything. Given that
that's allowed by clutter_actor_paint(), the paint volume should also
follow here and return an empty paint volume in case the custom clip is
empty.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1592>
b5a7fe4b23