compositor-wayland: fix shm_buffer damage init

compositor-wayland: fix shm_buffer damage init

It appears that wayland_shm_buffer::damage is in the global coordinate
space. Therefore initializing it to width x height at 0,0 is not correct
for any output not positioned at 0,0. That is, all outputs after the
first one get it wrong.

Initialize it from the output region, which is in the global coordinate
space.

While at it, add a comment to note that damage is in global coordinate
space. As I can see, this was the last confusion about it.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Acked-by: Daniel Stone <daniels@collabora.com>