Qt: Use QT_ENABLE_HIGHDPI_SCALING starting with Qt 5.14

[Megamouse]

The following was added with Qt 5.14:

• Applications can now opt-in to use non-integer scale factors, for example Windows at 150%. Use QGuiApplication::highDpiScaleFactorRoundingPolicy or QT_SCALE_FACTOR_ROUNDING_POLICY to set the rounding policy.
• Added the QT_ENABLE_HIGHDPI_SCALING environment variable which enables high-dpi scaling based on display DPI. Replaces QT_AUTO_SCREEN_SCALE_FACTOR (now deprecated), and corresponds to the Qt::AA_EnableHighDpiScaling application attribute.

So starting with Qt 5.14 I'm adding a command line argument "dpi-rounding" that accepts values in the valid range of Qt::HighDpiScaleFactorRoundingPolicy.