libweston: cancel idle_repaint on output destroy

If the idle_repaint() callback has been scheduled when a weston_output
gets destroyed, the callback will hit use-after-free. I have encountered
this when migrating the wayland backend to the head-based API, using
--sprawl, and closing/disconnecting one of the parent compositor
outputs.

Store the idle_repaint callback source, and destroy it in
weston_output_release(), ensuring we don't get a stale call to
start_repaint_loop later.

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