anchor.diff:    GraphicsDevice get graphicsDevice => Orange.instance.graphicsDevice;
     Camera camera;
     Color backgroundColor = new Color.fromHex(0x84A6EE);
  +  // TODO
  +  Color ambientColor = new Color.fromHex(0x0);
  +  
  +  // physics
  +  Vector3 _gravity;
     PhysicsEngine _physicsEngine;
     PhysicsEngine get physicsEngine => _physicsEngine;
     bool get physicsEnabled => _physicsEngine != null;
  
positive.code: part of orange;
  
  
  abstract class Scene {
    static int FOGMODE_NONE = 0;
    static int FOGMODE_EXP = 1;
    static int FOGMODE_EXP2 = 2;
  
negative.code:       _physicsEngine = null;
        return false;
      }
      if(gravity == null) gravity = _gravity;
      _physicsEngine._initialize(gravity);
      return true;
    }
  