libweston: make wl_output point to weston_head

The user data of a wl_resource representing a wl_output protocol object
used to be a pointer to weston_output. Now that weston_output is being
split, wl_output more accurately refers to weston_head which is a single
monitor.

Change the wl_output user data to point to weston_head.
weston_output_from_resource() is replaced with
weston_head_from_resource().

This change is not strictly necessary, but architecturally it is the
right thing to do. In the future there might appear the need to refer to
a specific head of a cloned pair, for instance.

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>