stable: add viewporter draft

This XML file has been copied verbatim from Weston 1.10.0 release,
protocol/scaler.xml.

The interfaces still need renaming according to wayland-protocols
policy. Also a redundant request needs to be removed. These will be done
in a follow-up patch to clearly show the changes.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
Acked-by: Bryce Harrington <bryce@osg.samsung.com>