[ODE] dTerrain aabb bug

[Geoff Carlton]

There's a subtle problem with dTerrainY and dTerrainZ that just caught 
me up.

Their calculate functions return an aabb bounded by the min and max 
height, however this can cause problems for flat terrains.  Fast moving 
objects may penetrate the terrain entirely and then skip the collision 
due to the aabb test.  The fix is to either return the aabb with some 
padding in the appropriate dimension, or just return that dimension as 
-infinity.

I'd submit this as a patch, but I'm not sure how to do that. :-)

Geoff