Test for total containment in BVTVisitor.

When checking for overlap with a volume (e.g. frustum, AABB), you can skip a lot of tests by saying "if an AABB is totally contained within the query volume, then immediately add all the leaves in the AABB". If you implement B+-style pointers in the leaves, you can skip even more steps by not having to walk the tree.

I think that this would greatly reduce the number of tests when frustum culling, although I'm not sure if it matters much.