Use the gl-renderer border code shared with headless-backend. We can
drop all this open-coded stuff.
In the output disable path, make sure to call this only when gl-renderer
is used. It will also reset the border state in gl-renderer, which is
harmless here, and it's necessary in the resize path.
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>