BVHHelper: Allow for recursive bounds visualization

[gkjohnson]

• Multiple BVH "roots" can be merged into a single geometry rather a new object being constructed for each root.
• If something like a "scene" bvh is created we should be able to continue visualizing the bounds of any BVHs on the leaves.

[gkjohnson]

One issue is that multiple objects my reuse the same BVH. In addition to being dynamic, the current strategy would also require duplicating the geometry data. Something like a "BatchedMesh" line renderer would be best.