protocol: allow to send a zero physical output size

Physical size doesn't always make sense for all outputs. In case
it's not available or not relevant, allow compositors to send zero.

Acked-by: Daniel Stone <daniels@collabora.com>
Acked-by: Jonas Ådahl <jadahl@gmail.com>
Acked-by: Olivier Fourdan <ofourdan@redhat.com>