/**
  * Handle all world logic. This involves updating entity positions, calculating collisions, etc.
  *
  * @param dt change in time since last frame (in seconds)
  */
 private void loop(float dt) {
   // We should call the world.step(float dt, int velocityIterations, int positionIterations)
   // recommended velocityIterations is 8 and positionIterations is 3
   // OR we could also call it in the render method down below.
   keyboard.handleKeyboard();
   sceneManager.update(dt);
 }