libweston: introduce weston_output_from_resource()

This is a simple wrapper for casting the user data of a wl_resource into
a struct weston_output pointer. Using the wrapper clearly marks all the
places where a wl_output protocol object is used.

Replace ALL wl_output related calls to wl_resource_get_user_data() with
a call to weston_output_from_resource().

v2: add type assert in weston_output_from_resource().

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Armin Krezović <krezovic.armin@gmail.com>