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.