Brushing

I've made some updates to accomodate different brush sizes and shapes, which wasn't hard as I'd planned ahead for it, mostly just some standard UI work to allow for brush editing. I think that the large brush sizes really show off the paint-like feel of things much better than the small brush sizes.

Brush shapes can also be modified (the screen shot only shows a circular brush) using predefined shapes or allowing customized ones. The brush editor isn't shown since it's not terribly interesting to look at.