[ODE] Damping

[Joe Ante]

> This looks wrong. If I step five times with time step 0.2
> using this method, I will get a different end velocity than
> if I step twenty times with time step 0.05. I believe you
> need to involve a powf() in the calculation of the actual
> timestep dependent damping to apply, if you apply it to
> velocity in this way. Alas, powf() is sadly quite slow :-(
> 
> If you don't want to involve the powf(), I'd suggest not
> scaling damping by timestep at all, and save both a multiply
> per body and the clamp (which may possibly branch, which may
> be more expensive than a divide).
Furthermore applying velocity directly is less robust than applying a
damping force directly.

Joe Ante