SphereGenerator

This isn't mergable yet, but I wanted to start a pull request to use as a communication point. I know you're working on the base class anyway, so I fully expect that this will need to evolve in step. :)

Happy to see mesh generation is going in, as it's one of the reasons libs like Three.js are so popular in my opinion. Pretty happy with the general structure, and I'm very fond of the Vector<N>Arrays for dealing with interleaved data!

I'm reasonably sure the vertex generation is correct, and the index data is either right or possibly has an off-by-one issue somewhere, but it's kind of hard to tell because at the moment the BoxGenerator in the sample isn't rendering correctly, so I'm not sure if the problem is me or the rest of the work-in-progress code. (That's what I get for building on a feature as said feature is being built. :P)

Anyway, I'm happy to hear comments, and I'll post a couple of my own as well.