window-actor/wayland: Ignore unmapped subsurfaces in direct scanout check

Clients expect us to ignore them, thus do so. Right now, if the topmost subsurface is an unmapped subsurface, it will fail the meta_surface_actor_is_opaque() check if it was never mapped before. Further more, unmapped subsurfaces would wrongly disable our "only one surface and fullscreen" optimization.

Closes #2550 (closed)