Adds (somewhat crappy) controls to example game

sky/examples/game/lib/sprite_box.dart

 part of sprites;
 
 enum SpriteBoxTransformMode {
   nativePoints,
   letterbox,
   stretch,
   scaleToFit,
   fixedWidth,
   fixedHeight,
 }
@@ skipping 19 matching lines @@
     transformMode = mode;
     _systemWidth = width;
     _systemHeight = height;
 
     _scheduleTick();
   }
 
   double get systemWidth => _systemWidth;
   double get systemHeight => _systemHeight;
 
+  TransformNode get rootNode => _rootNode;
+
   void performLayout() {
     size = constraints.constrain(Size.infinite);
   }
 
   void handleEvent(Event event) {
     switch (event.type) {
       case 'pointerdown':
         print("pointerdown");
         break;
     }
@@ skipping 80 matching lines @@
     // Count the number of frames we've been running
     _numFrames += 1;
 
     // Print frame rate
     if (_numFrames % 60 == 0) print("delta: ${delta} fps: ${1.0/delta}");
 
     _rootNode.update(delta);
     _scheduleTick();
   }
 }