Consider adding cast operators to System.Drawing types

[JeremyKuhne]

Is your feature request related to a problem? Please describe.
It is tedious and error prone (particularly with RECT/Rectangle) to convert between the native types and System.Drawing types (Point, Size, Rectangle, and *F variants).

Describe the solution you'd like
In Windows Forms we define the RECT, POINT, etc. structs with cast operators (often implicit). We also provide useful properties on RECT for convenience, notably .Width and .Height.

[elachlan]

@AArnott should they be added via templates or directly in the generator?