background-content: Apply Cullable clipping even in the transformed case

Just like we used to before 30809665.

Because in some cases `clip_region` is able to shave off an extra pixel
from the edge of the redraw rectangle(s). And not shaving that off was
making the background rendering inconsistent with shaped-texture, causing
occasional off-by-one artefacts. Now both shaped-texture and
background-content agree on the clip region again that doesn't happen.

Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1443

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1464