wayland: Make window opacity work

Under wayland, the compositor doesn't have a 'overall window alpha'
knob, we just need to add the alpha to the buffers we send.
Client-side alpha, if you want to call it that.

Implement this by reusing the existing alpha support for non-toplevel
widgets. As a side-effect of the implementation, windows with RGBA
visual under X will now also use per-pixel alpha, instead of
overall alpha.