window-actor: Add paint_to_content() function

Merged Robert Mader requested to merge rmader/mutter:window_actor_get_content into main

From the main commit:

Analogous to `get_image()` this returns a `ClutterContent` for a
given `MetaWindowActor`. This can be used to implement window
effects without a roundtrip from GPU to CPU memory.

And some cleanups. Should be merged together with gnome-shell!1879 (merged)

Required for gnome-shell!1954 (merged)

Edited by Robert Mader

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