Optimize Polyline Bounding Spheres

Computes tighter bounding spheres for polylines with the same material.

Before:
![image](https://f.cloud.github.com/assets/1494815/889780/d7945e40-fa25-11e2-926e-d1b5ffe41d43.png)

After:
![image](https://f.cloud.github.com/assets/1494815/889777/b37be0d2-fa25-11e2-99b7-c5882bdbba22.png)