window: Actually interpret the FRAME_SHAPE_CHANGED result

When I refactored this out into a vfunc, I forgot to change the
code that interprets the result flags to actually respect the
new FRAME_SHAPE_CHANGED result flag.

Since we weren't ever clearing the frame bounds, this meant that
the "shadow clip" wasn't ever updated as a result. Since right now
all Wayland surfaces are considered ARGB32, we always clip shadows
under frames, and thus shadows had this weird "punch-out" from the
first frame shape.
surface-actor-wayland: Fix meta_surface_actor_wayland_is_argb32

This prevents us from clipping shadows under windows for all
apps, and loads of other fun optimizations.