- Move header to base/include to make it applicable for base types
like 'Affinity' down the road.
- Represent 'Rect' as typle of point and area, which is the most
common form of initialization, creates in valid 'Rect' by default.
- Turn Point, Area, and Rect into compound types, making x, y, w, h, at,
area accessible without a method call
- 'Rect::Compound' function for constructing a 'Rect' from two points,
replacing a former constructor
- Use result type 'Rect::Cut_remainder' instead of out parameters.
Fixes#5239
c629c54153