Adds support for zOrder and references to parent nodes in sprites

sky/examples/game/lib/sprite_box.dart

Index: sky/examples/game/lib/sprite_box.dart
diff --git a/sky/examples/game/lib/sprite_box.dart b/sky/examples/game/lib/sprite_box.dart
index 40ca275110064029317a44937f95b50e775dc389..62e983a432b17a2c1cd4c342c54785311cede0fa 100644
--- a/sky/examples/game/lib/sprite_box.dart
+++ b/sky/examples/game/lib/sprite_box.dart
@@ -23,9 +23,15 @@ class SpriteBox extends RenderBox {
   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;
@@ -34,6 +40,13 @@ class SpriteBox extends RenderBox {
     _scheduleTick();
   }
 
+  void _addSpriteBoxReference(TransformNode node) {
+    node._spriteBox = this;
+    for (TransformNode child in node._children) {
+      _addSpriteBoxReference(child);
+    }
+  }
+
   double get systemWidth => _systemWidth;
   double get systemHeight => _systemHeight;