libweston: use head in wl_output global

As a wl_output represents weston_head, use a weston_head pointer as the
wl_output global's user data.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
v5 Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Ian Ray <ian.ray@ge.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Acked-by: Derek Foreman <derekf@osg.samsung.com>