compositor-x11: migrate to head-based output API

Follow the standard pattern set by the headless backend which also uses
the the window output API.

Stops relying on the implicit weston_output::head.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Ian Ray <ian.ray@ge.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Acked-by: Derek Foreman <derekf@osg.samsung.com>