Allow to add additional modes to outputs
Created by: agx
This adds support for adding additional modes to an output. This can be useful when a user wants to define custom modes (similar to xrandr --newmode).
This will become more even more interestin when we want to allow clients to add custom modes e.g. via https://github.com/swaywm/wlr-protocols/issues/15
Fixes #1080 (closed)