[ODE] Opcode TrisMesm collisions

[Pierre Terdiman]

> Is "double-sided collisions" hard to code into OPCODE ? :)

It's already there.

OPCODE only does "collision detection", not "contact generation". I report
all triangles touched by, say a sphere, regardless of winding - since the
sphere and triangles would collide anyway, in both CW or CCW mode.

For ray-mesh you even have a "culling" boolean parameter to tell the system
to backface cull the faces or not.

So the problem could be in the contact generation part instead, i.e. the
code contributed by Erwin.

Pierre