clutter-actor: Clip the current draw framebuffer

`clutter_actor_set_clip*` was being ignored for actors inside offscreen'd
actors because offscreening uses a different framebuffer.

Let's use the current draw framebuffer (which is always right) instead of
assuming the current draw framebuffer is that of the stage. Same as we
did in b8340f13.

Closes: GNOME/gnome-shell#1078
GNOME/mutter!516
3 jobs for !516 with fix-offscreen-clip in 7 minutes and 4 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
  Review
passed #257442
check-commit-log

00:01:03

 
  Build
passed #257443
build-mutter

00:01:41

 
  Test
passed #257444
test-mutter

00:04:18