This implements the basics of the new screenshooting protocol. The
actual pixel operations will be implemented separately in the renderers
and DRM-backend.

See the previous commit "protocol: new screenshooter protocol" for why.

If DRM-backend needs more from weston_capture_task when it implements
writeback screenshooting, it will be easy to add user_data or expose
weston_capture_task::link for the backend to use. Those were not added
yet because it is uncertain what is actually needed.

The DRM-backend no-damage optimization requires special handling here as
well. See also 7f1a113c .

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