Invoked when two physics objects of the registered group are about to
collide. invoked on the physics thread.
This is only invoked when the collision will happen based on the
collisionGroup and collideWithGroups settings in the
PhysicsCollisionObject. That is the case when one of the parties
has the collisionGroup of the other in its collideWithGroups set.
nodeA - collision object #1
nodeB - collision object #2
true if the collision should happen, false otherwise