Reduce usage of geometry types from System.Drawing namespace

[anders9ustafsson]

To facilitate the adaptation to portable class libraries, usage of geometric types from the System.Drawing namespace (e.g. Point, SizeF, Rectangle) should be kept at a minimum. Usage should be avoided completely in the public API. Ideally, the System.Drawing geometry types should only be used when required by the signature of a called method and there is no available alternative. Cases where this might occur is when calling e.g. Graphics.DrawImage and similar methods.

There is already a suitable replacement for System.Drawing.Point in the Dicom.Imaging.Mathematics namespace, namely Point2. Types analogous to Point2 may be implemented to cover up for other System.Drawing geometry types to maximize the portability of the library.