[panelview] Avoid assert if max < min
Max and min are calculated from two completely different sources. There's scope for them to not be the expected way round.
We have an assert reported on this line which must mean a subtle qBound -> std::clamp porting difference.
BUG: 454064
This is a completely blind fix I made no effort to reproduce.
-- Also make both horizontal and vertical use the same algorithm, I don't know how one ended up ported the other not.