WIP: Add opaque region culling to Clutter
Add some infrastructure to Clutter which allows getting opaque regions of ClutterContents and build unobscured regions based on those and the background color of actors. Also take clips into account. Proper API for effects will follow later...
Also expose the opaque regions of MetaBackgroundContent and MetaShapedTexture to Clutter using the new API.
The interesting stuff starts at commit 1db1c3b1, everything before are dependencies.
Depends on !1498 (merged)
Also depends on this cairo change: https://gitlab.freedesktop.org/cairo/cairo/-/merge_requests/40