[ODE] Heightfield Origin Change

[David Walters]

Hi, I've just submitted a change to SVN which moves the origin of the
heightfield to it's centre. As there were a few people who didn't
agree with it being in this location, I have added a local build
option for them. If you #define DHEIGHTFIELD_CORNER_ORIGIN then the
position of the geom will locate the (0,0) corner as before.

I hope this work for all combinations, I've tested it a fair bit, but
something stupid might have crept in, especially with the
non-placeable mode.

Regards,
Dave