Replace all the remaining weston_output::current_mode and borders[] uses
with the fb_size and the compositing area. The result is the same, but
we stop depending on weston_output, and border texture sizes which may
not be the same as border sizes.

This is more correct, semantically.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>