[ODE] small sized capsule on heightmap problem

[David Walters]

> The terrain is infinitely thin. This means that objects can tunnel
> through terrain, if they move quickly in a single time step. The
> solution is to make the objects bigger, to take much smaller time steps,
> or to do some kind of continuous collision (like casting a ray or
> capsule between old and new position each step).

Actually, although this may be the case (I have not tried to reproduce
any tunneling) - it is actually not intentional. The terrain has a
'thickness' parameter added to form a cuboid base attached at the
lowest vertical position and it is intended for contact points behind
any of the tiled plane sections to count up to this depth - the AABB
display in test_heightfield should demonstrate this.

I'm not sure how you would improve the heightfield collider to fix
this problem, but that is the desired operation never-the-less.

Regards,
Dave