[Development] The QT_SCREEN_SCALE_FACTORS rounding problem is still annoyingly valid

[Ilya Fedin]

This is a follow-up to the thread I started a year ago. The fix was
momentally reverted back then leading to me patching Qt all that time. I
still get reports from users using distribution packages as I obviously
can't patch their Qt. The application is just unusable for those users
using KDE's fractional scaling. I believe another attempt to fix this
should be made, I've re-reported the bug as
https://bugreports.qt.io/browse/QTBUG-110626. I copied the patch I
proposed in the previous report, why not to take this approach? Unlike
https://codereview.qt-project.org/c/qt/qtbase/+/412296, it doesn't
change the setScreenFactor method and shouldn't lead to the problems it
was reverted due to.