weston-info: display output scale

Fall back to not showing anything as before if we don't have a
compositor with wl_output new enough (version 2 or newer).

Signed-off-by: Jonny Lamb <jonny.lamb@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>