GaugeVario: use Point2D<int>
GaugeVario: use int instead of PixelScalar
GaugeVario: convert enum to constexpr
GaugeVario: move degrees_per_unit back into ValueToNeedlePos()

The previous commit broke the linker on FIXED=y.
Dialogs/ComboPicker: use int instead of PixelScalar
Form/ButtonPanel: use int instead of PixelScalar