Allow to set additional output modes
This will allow us to e.g. set the phone's geometry in the VM images.
See https://github.com/swaywm/wlroots/pull/1095 and https://github.com/swaywm/wlroots/issues/1080 for additional motiviation. Since this doesn't look as it will go in upstream easily let's carry it in our fork for the moment.