window-actor: Add paint_to_content() function
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