[ODE] Heightfield Origin Change

[Justin Couch]

David Walters wrote:

> I wanted to be responsible and make a change that didn't completely
> break ODE heightfields for the few people who might be using them in
> this old way, however the way they work now should be thought of as
> the standard as it is not planned to change again.

One thing to note is that conventionally heightfields all start at the 
corner. Of the dozen or so different file formats that I can think of 
off the top of my head, none of them start with the origin in the 
center. They all use a corner as it is far more natural to describe them 
that way as corner then grid from there. Describing from a "center" 
position means the center may or may not line up with the grid depending 
on whether there is odd or even number of rows or columns.

Justin