Places demo game in a OneChildRenderObjectWrapper and adds a star field and nebula for background

sky/examples/game/lib/game_demo_box.dart

 part of game;
 
 const double _steeringThreshold = 0.0;
 const double _steeringMax = 150.0;
 
 class GameDemoBox extends SpriteBox {
 
   GameDemoBox(GameDemoWorld game) : super(game, SpriteBoxTransformMode.letterbox);
 
   GameDemoWorld get _gameWorld => this.rootNode;
@@ skipping 50 matching lines @@
         case 'pointerup':
         case 'pointercancel':
           if (pointer == _firstPointer) {
             // Un-assign the first pointer
             _firstPointer = -1;
             _firstPointerDownPos = null;
             _gameWorld.controlSteering(0.0, 0.0);
           }
           else if (pointer == _secondPointer) {
             _secondPointer = -1;
-            _gameWorld.controlFire();
           }
           break;
         default:
           break;
       }
     }
   }
 
 }