protocol: unify wl_viewport src and dst size rules

protocol: unify wl_viewport src and dst size rules

Let's make the source and destination size rules consistent: neither can
have zero, {-1, -1} disables it, and other negatives are not allowed.

The sanity of allowing zero sized source rectangle as debatable. Now the
minimum becomes 1/256x1/256, and with output_scale the actual samples
may be even smaller. That should be enough.

On not allowed values, raise a protocol error. This should help catch
bugs in clients that accidentally send garbage values.

The old wl_viewport.set request remains the same, and can still produce
zero sized source rectangle.

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