anchor: World:update
  Update the state of the world.
  World:update( dt, velocityiterations, positioniterations )
  dt number The time (in seconds) to advance the physics simulation.
  velocityiterations (8) number The maximum number of steps used to determine the new velocities when resolving a collision.
  positioniterations (3) number The maximum number of steps used to determine the new positions when resolving a collision.