Adds basic sprite sheet support to sprites

sky/examples/raw/spinning_image.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'dart:math' as math;
 import 'dart:sky';
 import 'package:sky/mojo/net/image_cache.dart' as image_cache;
 
 double timeBase = null;
 
 Image image = null;
 String url1 = "https://www.dartlang.org/logos/dart-logo.png";
 String url2 = "http://i2.kym-cdn.com/photos/images/facebook/000/581/296/c09.jpg";
 
 void beginFrame(double timeStamp) {
   if (timeBase == null) timeBase = timeStamp;
   double delta = timeStamp - timeBase;
   PictureRecorder canvas = new PictureRecorder(view.width, view.height);
   canvas.translate(view.width / 2.0, view.height / 2.0);
   canvas.rotate(math.PI * delta / 1800);
   canvas.scale(0.2, 0.2);
   Paint paint = new Paint()..color = const Color.fromARGB(255, 0, 255, 0);
+
+  // Draw image
   if (image != null)
     canvas.drawImage(image, -image.width / 2.0, -image.height / 2.0, paint);
+
+  // Draw cut out of image
+  canvas.rotate(math.PI * delta / 1800);
+  if (image != null) {
+    var w = image.width.toDouble();
+    var h = image.width.toDouble();
+    canvas.drawImageRect(image,
+      new Rect.fromLTRB(w * 0.25, h * 0.25, w * 0.75, h * 0.75),
+      new Rect.fromLTRB(-w / 4.0, -h / 4.0, w / 4.0, h / 4.0),
+      paint);
+  }
+
   view.picture = canvas.endRecording();
   view.scheduleFrame();
 }
 
 void handleImageLoad(result) {
   if (result != image) {
     print("${result.width}x${result.width} image loaded!");
     image = result;
     view.scheduleFrame();
   } else {
@@ skipping 13 matching lines @@
 
   return false;
 }
 
 void main() {
   image_cache.load(url1, handleImageLoad);
   image_cache.load(url1, handleImageLoad);
   view.setEventCallback(handleEvent);
   view.setBeginFrameCallback(beginFrame);
 }