Draft: use overlay planes to do direct scanout more often

This makes direct scanout work when there's some overlay above a fullscreen window, like for example a picture-in-picture video over a game. For testing, this also ignores server side shadows, so you also get direct scanout with the volume OSD, or even wallpaper + panel.

For details, see the individual commits.

Draft because

• it contains !6066 (merged) and !6203 (merged) and those should be merged first
• for easier testing, it also contains !6193 (merged)
• we need better effects integration than "blocks direct scanout". Blur and contrast need to be able to block direct scanout on the Item they affect
• this should be tested on various hardware before merging
• there should be an environment variable to disable the overlay plane usage
• before merging, the check for server side shadows needs to be uncommented
• some color management checks are still missing
• screen casting needs to compose all the non-cursor layers?