Draft: xwayland: Add a fixed geometry size for rootfull

When running rootless as well as rootfull, Xwayland gets its outputs configuration from the Wayland compositor.

When running rootfull, it means that we end up with a large black surface the size of all monitors combined, that's not very convenient and there is no way for set the desired size of the Xwayland window.

Add a new command line option "-geometry" to force a specific mode when running rootfull for the user to specify the root window size to use for Xwayland.

That option has no effect when Xwayland is running rootless.

Signed-off-by: Olivier Fourdan ofourdan@redhat.com Closes: #1338 (closed)