xwayland: add a fixed geometry size for rootful

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

When running rootful, 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 rootful for the user to specify the root window size to use for
Xwayland.

That option has no effect when Xwayland is running rootless.

v2: Not using libxcvt as the mode may not be a valid CVT mode.
v3: Add a set of XRandR modes and the RR hooks to make that work.
    Update the man page for Xwayland.
v4: Add RandR 1.0 support for older clients
v5: Fix XVidMode failing with a BadMatch
v6: Add a separate xwl_output specifically for fixed mode, instead of
    using the existing output list - that will allow for further
    improvements like a fullscreen mode eventually.
v7: Sort the RR modes
v8: Fix RandR 1.0
v9: Add physical size
v10: Cleanup

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Closes: #1338