Fix rendering of debug bounding volumes in 2D and CV

We were using a unit sphere and then deforming it with a model matrix. This does not work in 2D and CV modes because we do not support setting the Primitive's modelMatrix property in those modes (because it would require us to recompute the geometry to handle dateline breaks).

The simple solution is to recreate a primitive every frame, which is slower but does not matter much when debugging.

Now that we can render these properly, it seems to expose several other issues, namely that the sphere appear wrong in many cases.  I don't think this is an error in the debug bounding volume rendering, but instead an issue with the volume itself.  Any ideas?

Try this:

1. Open vehicle.czml with Cesium inspector enabled
2. Select a billboard and turn on bounding sphere
3. They seem incorrect in 2D and CV.

3D:
![image](https://f.cloud.github.com/assets/1495196/2414179/4381e966-aae7-11e3-8071-48777f804d24.png)

2D:
![image](https://f.cloud.github.com/assets/1495196/2414187/559b83fa-aae7-11e3-943b-7004665fd1de.png)

CV:
![image](https://f.cloud.github.com/assets/1495196/2414178/38124b5c-aae7-11e3-9f3d-3403e80d2d83.png)