Style and performance tweaks

The changes I've made relate to what I wrote originally in a comment on the Reddit thread.

  * I've cut down a lot of the work that was being done on each animation frame. The canvas is touched less often, and that made a significant difference. Some of the computations were sped up a little too, especially around the heavily-used classes.

  * The "settings" stuff is cleaned up a little, and the updates are no longer done on every frame (really expensive to mess with the DOM!).

This is a really fun toy, and now it runs noticeably faster in Firefox. Feel free not to merge; I just wanted you to be able to see example changes.