protocol,compositor: split wl_viewport setters

protocol,compositor: split wl_viewport setters

Bump wl_scaler and wl_viewport versions to 2. Add new requests
wl_viewport.set_source and .set_destination, which are meant to replace
wl_viewport.set request.

Now a client can set and unset just one of source rectangle and
destination size. Define the semantics when one of these is unset.

Implement these semantics changes in compositor and pixman renderer.
GL-renderer does not need changes.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>