It is only useful for clients that do not set an opaque region but
still can be detected as being opaque. This is helpful for X11 clients
as opaque regions only got introduced around 2012 and only as part of EWMH
and are thus not used in many cases.
On Wayland however opaque regions have been part of the core protocol from the
beginnig and we can assume they are used more commonly.
As the current implementation in `MetaWindowActor` does not handle Wayland
subsurfaces well, instead of adding more complexity just move it to
`MetaWindowActorX11`.
While on it, take the shape region into account that is set when clients
use the X Nonrectangular Window Shape Extension Protocol, so we have exact
culling with those clients.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/1058
f5a28aa9e4