backends/drm: support configuring custom modes
Depends on libraries/plasma-wayland-protocols!121 (merged). Also see libkscreen!266 (merged)
Only caveat with this is that you can't add a custom mode and switch to it atomically, but I don't think it's a big issue to do one and then the other. It could also be fixed with the protocol later as well (having some flag on the mode that it should become the current one, or something like that).
This also introduces a reduced blanking flag for modes, which means the KScreen UI could be more easily extended to show that information (to differentiate between otherwise identical modes).
Edited by Xaver Hugl