compositor-drm: Use signed int for width/height

This makes it sign-compatible with weston_output->{width,height}.

Differential Revision: https://phabricator.freedesktop.org/D1486

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Armin Krezović <krezovic.armin@gmail.com>
1 file changed