This is called after a contact is updated. This allows you to inspect a
contact before it goes to the solver. If you are careful, you can modify the
contact manifold (e.g. disable contact).
A copy of the old manifold is provided so that you can detect changes.
Note: this is called only for awake bodies.
Note: this is called even when the number of contact points is zero.
Note: this is not called for sensors.
Note: if you set the number of contact points to zero, you will not
get an EndContact callback. However, you may get a BeginContact callback
the next step.
Note: the oldManifold parameter is pooled, so it will be the same object for every callback
for each thread.