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.