DO NOT MERGE AS-IS
For some extra-high DPI displays even higher settings will are needed. E.g. 13" 4k laptops (there are many of these now) need 3x scale, 15" 4k ones 2.5x. Or desktop monitors, like Dell's 32" 8k monitor is 2.5x, 2.75x or 3x.
Was uploaded so someone else could test the rest of the underlying stuff that's merged.
It runtime requires Qt 5.12 for xdg-output which the shell needs for fullscreen, which means we can merge for Plasma 5.16
Could this be merged, please? Fractional scaling is one of few things holding people back from using Wayland (including me).
Also, could it be a slider, like in X11? I prefer 1.8 scaling, but this PR only adds the options to set 1.0, 1.25, 1.5, 1.75 and 2.0 scaling. It would be great if it was more flexible.