Only integer scaling factors are supported without environment variable

When applying high DPI scaling, only integer scaling factors are supported unless explicitly overriding with environment variables. For example, on KDE Plasma when applying a global scale of 125% all menus and text are 2x normal size, much larger than the rest of the system.

Looking at the Qt Creator code, it appears to default the HighDpiScaleFactorRoundingPolicy value to Round and switch it to Floor if high DPI isn't enabled. In order to have proper scaling, the QT_SCALE_FACTOR_ROUNDING_POLICY=PassThrough environment variable must explicitly be set.

I would propose that PassThrough would be used by default if high DPI is enabled to ensure consistency across the system.