The SAH split would happily repeatedly split on the same axis
as long as the surface area was reduced as much as splitting
on the other axes. This resulted in sliver-like bounding boxes
for some scenes, which is terrible for the light tree.
The SAH splitting code now accounts for the diagonal of the
bounding box, favoring smaller ones. This seems to work well,
fixing the issue without introducing any apparent performance
regressions.
