ModelProcessor Unit Test

I added a unit test for `ModelProcessor` and in the process fixed a few bugs it exposed.

 - Fixed `BoundingSphere.CreateFromPoints` to match XNA results.
 - Fixed missing exception in `BoundingSphere.CreateFromPoints`.
 - Fixed NaN bounding sphere on empty `ModelMeshPartContent`.
 - Fixed `MaterialContent.GetTexture()` to not throw not found exceptions.
 - Added better test for missing vertex texture coordinates.
 - We no longer leak vertex color material settings to meshes without vertex colors.

The unit test now runs successfully on both XNA and MonoGame.  I hope to see this unit test expand as we fix problems with MonoGame's model processor.