[ODE] Improved plane AABB

[David Walters]

Thanks very much for this contribution, I've just integrated it in
SVN. I actually though this had been done before, but it was merely
mentioned in the comments of the function as a todo. Well spotted and
thanks again.

It actually seems to improve the 'test_moving_trimesh' example to the
point where it's _visibly_ quicker with this optimisation!

Regards,
Dave