[ODE] solving penetrations "statically"

[Erin Catto]

I didn't have time to check the patch, but I updated my 2D physics engine
Box2D. This engine uses sequential impulses, which are mathematically
equivalent to PGS. Here's how I did it:

- Each body has a bias velocity that is only used for correcting overlap.
- A separate bias impulse is accumulated for each contact.
- The bias velocity and impulse are initialized to zero each time step (no
warm starting).

The bias impulse and velocity are completely independent of the regular
impulses and velocity, so they could be solved in parallel. However, it was
convenient to solve them together and that should be more cache friendly.

You can get the code here: http://www.gphysics.com/files/Box2D.zip

You can switch between the two methods by using the macro
BIAS_PRESERVES_MOMENTUM in Arbiter.cpp. With the split velocity I was able
to use a large bias factor (0.8). This means that overlap is reduced 80% per
step. Such a large bias factor leads to serious bouciness with the merged
velocity.

I also tried using a split velocity for joints. The results were not nearly
so nice, especially for my suspension bridge test. I think that particular
arrangement is more stable with some compliance in the joints.

Erin