Geometry Web Worker

Moves `Geometry` creation and batching to a web worker.

I'm opening this request to get some early feedback. There are two failing tests. The reflection built-in material and the draw debug bounding sphere tests fail. The material test is a bug that can be reproduced in master and I'm opening an issue for it. I haven't decided the best way to fix the other test yet.

#1021 should be reviewed and merged before this.