/**
  * 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);
 }
 @Override
 public void render() {
   float dt = Gdx.graphics.getDeltaTime();
   loop(dt);
   Gdx.gl.glClearColor(0, 0, 0, 1);
   Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT); // illustration of basic drawing
   sceneManager.render();
 }