Add Bresenham-style ellipse drawing algorithm

[simonvarey]

Hi @expenses, thanks for the crate, it's been really helpful.

I needed to implement an ellipse drawing algorithm for one of my projects and I thought I'd upstream it, in case you were interested. Alternatively, it's here now in case anyone else needs it.

Let me know if you'd like me to make any changes.

[expenses]

Hi. Good job on this implementation. I'm still thinking about whether I'll merge this, as this crate and repository is more-or-less archived.