Adds support for zOrder and references to parent nodes in sprites

sky/examples/game/lib/sprite_box.dart

 part of sprites;
 
 enum SpriteBoxTransformMode {
   nativePoints,
   letterbox,
   stretch,
   scaleToFit,
   fixedWidth,
   fixedHeight,
 }
 
 class SpriteBox extends RenderBox {
 
   // Root node for drawing
   TransformNode _rootNode;
 
   // Tracking of frame rate and updates
   double _lastTimeStamp;
   int _numFrames = 0;
 
   SpriteBoxTransformMode transformMode;
   double _systemWidth;
   double _systemHeight;
 
   SpriteBox(TransformNode rootNode, [SpriteBoxTransformMode mode = SpriteBoxTransformMode.nativePoints, double width=1024.0, double height=1024.0]) {
+    assert(rootNode != null);
+    assert(rootNode._spriteBox == null);
+
     // Setup root node
     _rootNode = rootNode;
 
+    // Assign SpriteBox reference to all the nodes
+    _addSpriteBoxReference(_rootNode);
+
     // Setup transform mode
     transformMode = mode;
     _systemWidth = width;
     _systemHeight = height;
 
     _scheduleTick();
   }
 
+  void _addSpriteBoxReference(TransformNode node) {
+    node._spriteBox = this;
+    for (TransformNode child in node._children) {
+      _addSpriteBoxReference(child);
+    }
+  }
+
   double get systemWidth => _systemWidth;
   double get systemHeight => _systemHeight;
 
   TransformNode get rootNode => _rootNode;
 
   void performLayout() {
     size = constraints.constrain(Size.infinite);
   }
 
   void handleEvent(Event event) {
@@ skipping 85 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();
   }
 }