frames: Hackily adapt to GTK+ drawing changes

mutter is quite bad at using GTK+ correctly, relying on dumb things
like the single-buffering stuff. Hack up a temporary fix for the
newer GTK+ rendering changes.
frames: Don't bother painting the background

GDK (and also GTK+!) do this for us already.
frames: Simplify the clip region calculation

Since GTK+ already clips to the extended region for us, there's no need
to combine the two. This does lose the fast-path, but I don't actually
expect this to fire, as when we're composited, we really won't ever get
partial exposes.