compositor: fix wp_viewport.set_source errors
compositor: fix wp_viewport.set_source errors

The revised wp_viewport spec requires that unset has to have all of x, y,
width and height -1 to be recognized.

Check for negative x and y and raise the required error. The error event
now mentions the wl_surface, too.

Signed-off-by: Pekka Paalanen <0f6ae7001e34c37b02fb2c1c2573ddf8f442136a@collabora.co.uk>
Reviewed-by: Bryce Harrington <bdb14cfc1ab8d24e13de84adc40b41ae60c13bd1@osg.samsung.com>

sir-murray/weston,krezovic/weston,sir-murray/weston,krezovic/weston