surface-actor: Simplify culling logic

This reverts the commits 372d73e2 and 1d200452 - the special
case for alpha-less textures could only happen on Wayland, but now
the opaque region is also set in those cases.

This commit saves us some allocations, simplifies the logic a bit and
makes sure culling uses the same opaque region as our painting paths.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1463