[ODE] dHeightfield collider

[Martijn Buijs]

Hi,

I'm working on a general purpose heightfield collider, based on the "Terrain & Cone" contrib. It
removes the terrain dimension limitations (supports non-square fields & grid spacing, NPOT dims) and
allows deformable terrains (it should work great for animated water surfaces as well -- the reason I
named it "dHeightfield" rather than "dTerrain"). A bunch of fixes are included (AABB patch, callback
patch), and the geom creation interface has been changed to mimic the trimesh collider.

However, I'm not sure how I can make it to work with Z-up systems (I re-used dTerrainY code) without
duplicating code, and in an elegant, efficient fashion. Any suggestions?

If it proves stable (which so far it has) I think this collider would make a nice addition for the
next stable release. I'll do some more testing, write a readme etc. and try to post the code after
the weekend. :)

Martijn