wayland/actor-surface: Always set opaque region on alpha-less textures

Wayland clients using buffers without alpha channel are not expected to
set an opaque region. However, we rely on the opaque region for the fast
painting path in `MetaShapedTexture`.

Thus, make sure to always set an opaque region internally in those cases.
For X11 clients, wo do so already.

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