xwayland: make the output serials belong to the screen

Xwayland uses an output serial number it increments each time a new
Wayland output is added.

On server regeneration, that static value is not cleared, and therfore
the output numbers keep increasing each time the Xserver restarts.

To avoid that issue, make the output serial part of the xwl_screen,
which gets recreated on server regeneration, so that index is reset to 0
automatically on server regeneration.

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