weston: use wet.compositor consistently in main()

Rename user_data to wet, because it is called wet everywhere else.

Drop the local variable ec, because that is available as wet.compositor.

This models a little better that wet_compositor owns weston_compositor,
and not the other way around.

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