[ODE] Heightfield Origin Change

[David Walters]

> A separate build option is lame and leads to fragmentation and harder to
> test code.

This is true, however it's not a global build option - just an
optional hack you can make to 'heightfield.cpp' for if you absolutely
must have the corner as the origin.

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.

Regards,
Dave