[ODE] get triangle index of a trimesh

[Geoff Carlton]

As discussed a while ago, ODE really needs the indexes stored in the 
dContactGeom struct. 

I have implemented this (I called them side1, side2), with the integer 
meaning the index for trimeshes, box side for boxes, and -1 for 
everything else.  I was only interested in trimesh-*, so I didn't bother 
trying to return the box sides for box-box, leaving that one 
unimplemented.  Of lesser importance, capsules and cylinders could also 
return 1,2,3 depending on their closest surface (top, middle, bottom), 
and of course cones with 1,2 for (top, bottom).

Now I've untangled offsets, it should be easier to make up a patch for 
this one.  Unfortunately I'm fairly busy at the moment so its a low 
priority.

Geoff