Fixes matrix transformations in sprites

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);
-
-      // 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.Mode(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));
-    }
-  }
-  
-}