[ODE] dSetZero

[Jan]

I was just profiling ODE to see if it could be speeded up, and it turns out
it spends more time in dSetZero than anywhere else in my entire game!

Now it could be speeded up by calling memset instead, but it seems a lot of
these calls are superfluous (some having comments saying 'hmmm, this info
gets overwritten straight away anyway, so why bother?')
I could go through and remove them all by simple trial-and-error, but it
seems to me like someone who knows how ODE works 'on the inside' could do it
a lot faster and we'd all end up with quicker physics, rather than just me!
--
Jan