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.
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