Playable demo game and bug fixes in sprites

sky/examples/game/lib/game_box.dart

-part of game;
-
-const double _steeringThreshold = 20.0;
-const double _steeringMax = 50.0;
-
-class GameBox extends SpriteBox {
-
-  GameBox(GameWorld game) : super(game, SpriteBoxTransformMode.letterbox);
-
-  GameWorld get _gameWorld => this.rootNode;
-
-  // Handle pointers
-  int _firstPointer = -1;
-  int _secondPointer = -1;
-  Vector2 _firstPointerDownPos;
-
-  void handleEvent(Event event, BoxHitTestEntry entry) {
-    if (event is PointerEvent) {
-      Vector2 pointerPos = new Vector2(event.x, event.y);
-      int pointer = event.pointer;
-
-      switch (event.type) {
-        case 'pointerdown':
-          if (_firstPointer == -1) {
-            // Assign the first pointer
-            _firstPointer = pointer;
-            _firstPointerDownPos = pointerPos;
-          }
-          else if (_secondPointer == -1) {
-            // Assign second pointer
-            _secondPointer = pointer;
-            _gameWorld.controlThrust(1.0);
-          }
-          else {
-            // There is a pointer used for steering, let's fire instead
-            _gameWorld.controlFire();
-          }
-          break;
-        case 'pointermove':
-          if (pointer == _firstPointer) {
-            // Handle turning control
-            double deltaX = pointerPos[0] - _firstPointerDownPos[0];
-            if (deltaX > _steeringThreshold || deltaX < -_steeringThreshold) {
-              double turnForce = (deltaX - _steeringThreshold)/(_steeringMax - _steeringThreshold);
-              if (turnForce > 1.0) turnForce = 1.0;
-              if (turnForce < -1.0) turnForce = -1.0;
-              _gameWorld.controlSteering(turnForce);
-              print("steering: $turnForce");
-            }
-          }
-          break;
-        case 'pointerup':
-        case 'pointercancel':
-          if (pointer == _firstPointer) {
-            // Un-assign the first pointer
-            _firstPointer = -1;
-            _firstPointerDownPos = null;
-            _gameWorld.controlSteering(null);
-          }
-          else if (pointer == _secondPointer) {
-            _secondPointer = -1;
-            _gameWorld.controlThrust(null);
-          }
-          break;
-        default:
-          break;
-      }
-    }
-  }
-
-}