[gfx/style] Implement border-radius.

This patch is a first stab at implementing border-radius. It looks fine as long as
the border isn't an ellipse (that might not even parse yet), and the border-widths
around a border-radius are the same.

Here's a cool screenshot!

![](https://www.dropbox.com/s/gdtmgjrlnf82gzz/Screenshot%202014-11-12%2018.03.29.png?dl=1)

r? @pcwalton @SimonSapin