Set Qt::HighDpiScaleFactorRoundingPolicy::PassThrough also on Unix
Previously this was Windows-only, the reasons are described in
https://bugs.kde.org/show_bug.cgi?id=416078
However, one and a half years later I don't see any negative effects when setting this option, (In fact, I don't see any effects at all), both for X11 and Wayland. Let's therefore enable PassThrough for Unix as well, in order to reduce the amount of platform-specific code.