[ODE] Stabilizing cars (finally!)

[Gabriel Gambetta]

Thanks all for your comments.

I changed my implementation a bit. I always apply the force to the relative "down" direction of the chassis, but the magnitude is M.g.<Ua,Ul> where Ua is the absolute "up" vector ( (0, 1, 0) ) and Ul is the local "up" vector. No force is applied if <Ua,Ul> < 0. This still prevents the car from rolling, and eliminates some funny effects of my previous implementation (such as, the car would float if it ended up over its roof, or it would stick to vertical walls). This doesn't *completely* prevent the car from rolling (it's an arcade after all) and aerial acrobatics can still be done (loops are actually easier to do now).

While it would be great to know *why* this works, it's not a priority - it's an arcade game and I think I got the balance I wanted between realism and arcade driving.

	--Gabriel