Adds support for transfer mode in Paint and SpriteNode classes

sky/examples/game/lib/sprite_node.dart

 part of sprites;
 
 // TODO: Actually draw images
 
 class SpriteNode extends TransformNode {
   
   Image _image;
   bool constrainProportions = false;
+  double _opacity = 1.0;
+  Color colorOverlay;
+  TransferMode transferMode;
   
   SpriteNode() {
     this.pivot = new Vector2(0.5, 0.5);
   }
   
   SpriteNode.withImage(Image image) : super() {
     this.pivot = new Vector2(0.5, 0.5);
     _image = image;
   }
-  
+
+  double get opacity => _opacity;
+
+  void set opacity(double opacity) {
+    assert(opacity >= 0.0 && opacity <= 1.0);
+    _opacity = opacity;
+  }
+
   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));
+
+      // Setup paint object for opacity and transfer mode
+      Paint paint = new Paint();
+      paint.setARGB((255.0*_opacity).toInt(), 255, 255, 255);
+      if (colorOverlay != null) {
+        paint.setColorFilter(new ColorFilter(colorOverlay, TransferMode.srcATopMode));
+      }
+      if (transferMode != null) {
+        paint.setTransferMode(transferMode);
+      }
+
+      canvas.drawImage(_image, 0.0, 0.0, paint);
       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));
     }
   }
   
 }