xwayland: Add -force-xrandr-emulation switch

Adds a -force-xrandr-emulation cmdline switch that always exposes extra
modes when viewporter isn't exposed by the Wayland compositor.

Having the additional modes exposed by the X server is important for
games to function and be configured

Compositors, such as Gamescope (the compositor for Steam Deck),
support only a single window that is rendered in the centre of the
screen that is scaled up to fill the screen by the compositor based
on some user scaling settings.

Exposing viewporter, wouldn't make sense here, and could mislead native
Wayland clients, so exposing dummy modesets in X is preferred here.

Signed-off-by: Joshua Ashton <joshua@froggi.es>
Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>