window-actor/wayland: Allow direct scanout of translucent windows

Because starting in 43.1 (commit 909616b2) we enforce a black
background for see-through parts as noted by mutter#2520. So direct
scanout should (hopefully) be visually the same as compositing now
for non-opaque windows.

The idea here is to avoid punishing apps/games for choosing RGBA.
Although Draft until I find a workable test case and verify this
is presented as expected by KMS.
18 jobs for !2801 with direct-scanout-translucent in 29 minutes and 4 seconds (queued for 1 second)
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