Ellipse geometry

[duysqubix]

The geometry code covers:

• Points/Vectors
• Rectangles
• Circles
• Matrices

Lines (pairs of vectors) are covered by #167

Would it be useful to extend the circle code to instead be ellipses? I.E. a circular shape where the horizontal radius is potentially different to the vertical radius. I imagine the collision detection would be more complex than the other shapes.

I note that imdraw supports ellipse

Original issue: faiface/pixel#171