weston: store weston_compositor in wet_compositor

This makes it easier to just pass wet_compositor around and take the
weston_compositor from it.

It feels weird to go from weston_compositor to wet_compositor all the
time in internal functions. It's necessary in callbacks that cannot
carry wet_compositor, but otherwise it is awkward.

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>