windowManager: Use MetaWindowActor.paint_to_content() for window effects

windowManager: Use MetaWindowActor.paint_to_content() for window effects

Previously we used `get_image()`, which returned `cairo_surface`,
in order to create a `ClutterContent` with a screenshot of the
`MetaWindowActor`. This added a roundtrip from GPU to CPU memory.

Instead, use the new `paint_to_content()` API which lets us use a
`CoglTexture` directly as source of our `ClutterContent`.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1879>