If two modes are roughly the same, they should probably use the same UI
scaling factor. I.e. for the same monitor, if a 4K mode was configured to
have a certain scaling factor, and we generate a new configuration with
a similar sized 4K mode, we should re-use the scale previously
configured; however if we e.g. go from a 4K mode to a FHD mode, we
shouldn't.
This allows implementing better hueristics when using the switch-config
feature, where we'd be less likely to loose the for a certain monitor
mode combination previously configured scaling factor.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2479>
