Add safeguards to make it painfully obvious if we ever get the pairing
of wayland_backend_create_output_surface() and
wayland_backend_destroy_output_surface() wrong. Helps catching bugs.

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