No rush to do this now and I know it was already there, but I expect this to make a nice difference.

![image](https://cloud.githubusercontent.com/assets/782098/3098302/1c512346-e5ea-11e3-9ab8-5a30f1c74d9c.png)


It should be easy to cache `Math.tan(0.5 * fovy)` with the camera.