Augmented API for Rectangles, Points, Sizes

[migueldeicaza]

Currently we use a barebones implementation of those structures.

While we need to keep the structure fields in the same order, we should copy/paste the code for RectangleF and Rectangle into SKRect; PointF/PointI into SKPoint and so on.

[migueldeicaza]

Added the PointF/Point, perhaps some improvements can be done to Point3.

Pending: Rectangles and Sizes.

[mattleibow]

Added this in 17e456a