xwayland: add fullscreen mode for rootful

Add a new command line option "-fullscreen" to make the rootful Xwayland
window appear fullscreen.

This requires viewport support in the compositor and when used with
"-geometry" can emulate the full range of XRandR resolutions.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>