libweston: Clip damage to paint node visible region

commit 79212ba9 fixed a bug by
introducing a new one.

Before that point we could clip paint node damage to stale
visibility data. After that point we post damage for occluded
views, leading to large amounts of pointless drawing.

Add back the clip to visible region, in
weston_output_flush_damage_for_plane(), where we have up to date
visibility region information.

fixes 79212ba9

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>