Add BatchedTilesPlugin

[gkjohnson]

Fix #417

TODO

• Add cap to expansion - max vertex, index size? max number of expansions? Max instances?
  • If expansion cannot happen then just leave the geometry as-is.
• Account for cases where some geometry does not have the same attributes (ie no uvs) (load near tokyo, mt fuji)
• Support multiple instances / meshes per tile (google tiles can have multiple meshes per tile)

Later

• Afford ability to delete scene from tile nodes entirely
• Possibly support multiple batched meshes