Change triangle offsets to be relative to geometry range

[gkjohnson]

It will make generating and managing subsections of the arrays more manageable. Related to #513

[gkjohnson]

Once geometry can be "optimized" in BatchedMesh (ie the sub geometry ranges will be modified to collapse unused space) the MeshBVH triangle range references will be broken since they have moved. We need a way to "fix up" the built BVHs otherwise they will all have to be regenerated from scratch. There are a couple options for handling this:

1. "Fix up" the entire set of BVH nodes by adjusting each stored triangle "offset" field to be adjust for the new range. This should mostly mean adding / subtracting some amount each child triangle identifier. Updating every child node could be a bit slow, though.

2. Adjust all triangle identifiers to star at 0 and be relative to the specified BVH range when traversing. This means "fixing up" a BVH just means modifying the MeshBVH range but would mean having access to the geometry range in all traversal functions.

cc @agargaro you might be interested in this since you've been working on BatchedMesh stuff. Not sure if you have any other thoughts. I've made an issue to discuss the BatchedMesh "optimize" function here: mrdoob/three.js#29463