Add a Tetragon class.

This branch implements an unimplemented part of the Transform.transformRect function so it can apply a matrix transformation to a rectangle. This branch adds a new class 'Tetragon' derived from the Rect class and returns its instance instead of returning a Rect instance. (Applying a matrix transformation to a rectangle produces one of a rectangle, a parallelogram, a rhombus, or a square. That is, a transformed rectangle may not be a rectangle.)