Added a new bounding box type, BBox2, and changed BVH2 to use it.

BBox2 contains data for two bounding boxes, but is laid out in a SIMD
friendly way.  BBox2/Ray intersections are done against both contained
boundings boxes as once.

In practice, this doesn't appear to give much speedup, but it will be
useful reference for QBVH code in the future.