Adds basic touch handling to sprites and optimizes transformations

sky/examples/game/lib/sprite.dart

 part of sprites;
 
 // TODO: Actually draw images
 
 class Sprite extends NodeWithSize {
   
   Image _image;
   bool constrainProportions = false;
   double _opacity = 1.0;
   Color colorOverlay;
   TransferMode transferMode;
   
   Sprite() {
   }
   
   Sprite.withImage(Image image) {
     pivot = new Point(0.5, 0.5);
     size = new Size(image.width.toDouble(), image.height.toDouble());
     _image = image;
   }
 
   double get opacity => _opacity;
 
   void set opacity(double opacity) {
+    assert(opacity != null);
     assert(opacity >= 0.0 && opacity <= 1.0);
     _opacity = opacity;
   }
 
   void paint(PictureRecorder canvas) {
     canvas.save();
 
     // Account for pivot point
     applyTransformForPivot(canvas);
 
@@ skipping 29 matching lines @@
       canvas.drawImage(_image, 0.0, 0.0, paint);
     }
     else {
       // Paint a red square for missing texture
       canvas.drawRect(new Rect.fromLTRB(0.0, 0.0, size.width, size.height),
           new Paint()..color = const Color.fromARGB(255, 255, 0, 0));
     }
     canvas.restore();
   }
 }