gi3d obj bounding box not accurate for items that are entirely out of range

[rcoreilly]

e.g., the floor plane in example/gi3d -- at certain points it turns off b/c bounding box intersection goes to zero. some kind of projection math issue..

[rcoreilly]

needs actual frustum culling using actual frustum -- otherwise large planes need near plane intersection etc to properly compute bbox. this can be done by computing intersection with near plane to truncate large planes that go behind viewer. not necc at this point probably -- should test with layers though.