* xwayland-run, to spawn an X11 client within its own dedicated Xwayland
   rootful instance,
 * wlheadless-run, to run a Wayland client on a set of supported Wayland
   headless compositors,
 * and xwfb-run, a combination of the two other tools above which would
   be a replacement for xvfb-run specifically.

This is almost an entire rewrite.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>