It yields the objects that the ray needs to be tested against.
Thus it is the responsibility of the code using the iterator
to actually do the object-level ray tests and update the ray's
max_t etc. accordingly.
This keeps all of the BVH-related code generic with respect to
what kind of object/data the BVH actually contains, which means
the same BVH code can be used for both scene-level and
triangle-level data.
The BVH is now generic over any kind of data. The building
function takes in a closure that can bound the given data type
in 3d space, and the rest just works.
The code here is a bit messy right now. Just did enough to get
it working. But it needs to be cleaned up and report parse
errors in a human-readable way, among other things.
