Math/Point2D: add explicit casts to operator+

Eliminates narrowing warning.
OpenGL/Globals: use Point2D<unsigned> for screen size
Topography/XShapePoint: rebase ShapePoint on Point2D
Screen/Custom/Point: rebase RasterPoint on Point2D
GaugeVario: use FastIntegerRotation instead of FastPixelRotation
Screen/FastPixelRotation: remove unused class