I am an idiot: the quadtree code is now fixed.

tl;dr: needed more * by 2.

iab;wtd*: The index buffers on the GPU contain _two_ elements per object,
one for the top left of the object's bounding box, one for the bottom-right.
We were taking that into account when computing the number of indices to give
the DrawElements() call, but not when calculating the offset into the buffer.

Oops.

*: I am bored; want tedious detail