[ODE] ODE Octree culling

[Bram Stolk]

> Out of curiosity, how do other people handle this? Do you use one
> space for collisions and another for rendering? Your own space for
> both? Something else?

I have plib do my rendering, so I keep a separate plib
scene graph alongside the ode space. This requires me
to put ode geom in the ode space, and the equivalent
plib geometry in the plib scene. Also, each frame I have
to update the plib matrices.

   Bram