Adds support for transfer mode in Paint and SpriteNode classes

sky/examples/game/lib/sprite_node.dart

Index: sky/examples/game/lib/sprite_node.dart
diff --git a/sky/examples/game/lib/sprite_node.dart b/sky/examples/game/lib/sprite_node.dart
index ca6296a82d133880872b01accd2b8af498951110..586be71403b1c9975724df7c6b192bd3c4725c16 100644
--- a/sky/examples/game/lib/sprite_node.dart
+++ b/sky/examples/game/lib/sprite_node.dart
@@ -6,6 +6,9 @@ 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);
@@ -15,7 +18,14 @@ class SpriteNode extends TransformNode {
     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);

[Matt Perry, 2015/06/04 17:50:18]

space after <=

+    _opacity = opacity;
+  }
+
   void paint(PictureRecorder canvas) {
 
     if (_image != null && _image.width > 0 && _image.height > 0) {
@@ -37,7 +47,18 @@ class SpriteNode extends TransformNode {
       }
       
       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);

[eseidel, 2015/06/04 17:49:50]

Does dart put spaces around operators?  I think we have a dart format command
somewhere.  git cl format might also know how to format dart.

It feels like we want to do things like new Paint()..color =
colors.black.withOpacity(_opacity), but this is fine for now.

+      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 {