[ODE] better way to handle terrain

[Tri Sulistiono]

> > a single merged tri-mesh collision geom is faster than 'chunked'
> > tri-meshes...
> >
> > Tri S.
> >
>
> It works well because OPCODE automatically generates an AABB tree for each
> trimesh, which improves speed, just as if you split the terrain up
> yourself.
>
> -Chris
>

Certainly...

Initially I thought splitting the terrain tri-mesh will help speeding up the
collider a bit, i.e. by exploiting the early out bounding box test used by
the geom-geom collider internally... but the 'setup cost' of the BV-tree per
tri-mesh seems to actually slows it down, although I don't do much profiling
on this yet.

Merging all tri-meshes into single also help eliminates popping when we
crossing the border/junction between tri-meshes.

Tri S.