Adds basic support for images in game example

sky/examples/game/lib/sprite_node.dart

+part of sprites;
+
+// TODO: Actually draw images
+
+class SpriteNode extends TransformNode {
+  
+  Image _image;
+  bool constrainProportions = false;
+  
+  SpriteNode() : super() {

[abarth-chromium, 2015/06/01 21:58:55]

The call to super() is implied.  You don't need to write it.

+    this.pivot = new Vector2(0.5, 0.5);
+  }
+  
+  SpriteNode.withImage(Image image) : super() {
+    this.pivot = new Vector2(0.5, 0.5);
+    _image = image;
+  }
+  
+  void paint(PictureRecorder canvas) {
+
+    if (_image != null && _image.width > 0 && _image.height > 0) {
+      canvas.save();
+      
+      double scaleX = _width/_image.width;
+      double scaleY = _height/_image.height;
+      
+      if (constrainProportions) {
+        // Constrain proportions, using the smallest scale and by centering the image
+        if (scaleX < scaleY) {
+          canvas.translate(0.0, (_height - scaleX * _image.height)/2.0);
+          scaleY = scaleX;
+        }
+        else {
+          canvas.translate((_width - scaleY * _image.width)/2.0, 0.0);
+          scaleX = scaleY;
+        }
+      }
+      
+      canvas.scale(scaleX, scaleY);
+      canvas.drawImage(_image, 0.0, 0.0, new Paint()..setARGB(255, 255, 255, 255));
+      canvas.restore();
+    }
+    else {
+      // Paint a red square for missing texture
+      canvas.drawRect(new Rect.fromLTRB(0.0, 0.0, this.width, this.height),
+          new Paint()..setARGB(255, 255, 0, 0));
+    }
+  }
+  
+}