A rigid body.
A body definition holds all the data needed to construct a rigid body.
This is called when a body's shape passes outside of the world boundary.
Implement this class to provide collision filtering.
Implement this class to get collision results.
Implement and register this class with a b2World to provide debug drawing of physics
entities in your game.
Joints and shapes are destroyed when their associated
body is destroyed.
The world class manages all physics entities, dynamic simulation,
and asynchronous queries.