Optimize Geometry Bounding Sphere

Optimizes the bounding sphere for combined geometry.

Before:
![image](https://f.cloud.github.com/assets/1494815/890635/b07fe16c-fa39-11e2-8f9b-87420a38f388.png)

After:
![image](https://f.cloud.github.com/assets/1494815/890621/765ea180-fa39-11e2-842b-1983cd20c40d.png)