Adds basic support for images in game example

sky/examples/game/main.dart

+import 'dart:sky';
+import 'lib/game.dart';
+
+int numFrames = 0;
+double lastTimeStamp = null;
+
+PictureRecorder canvas;
+
+GameWorld root;
+
+void beginFrame(double timeStamp) {
+  // Calculate the time between frames in seconds
+  if (lastTimeStamp == null) lastTimeStamp = timeStamp;
+  double delta = (timeStamp - lastTimeStamp) / 1000;
+  lastTimeStamp = timeStamp;
+  
+  // Count the number of frames we've been running
+  numFrames += 1;
+  
+  // Print frame rate
+  if (numFrames % 60 == 0) print("delta: ${delta} fps: ${1.0/delta}");
+  
+  // Create a canvas to draw into
+  canvas = new PictureRecorder(view.width, view.height);
+  
+  // Update all the sprites
+  root.update(delta);
+  
+  // Draw all the sprites
+  root.visit(canvas);
+  
+  // Output the image to the screen
+  view.picture = canvas.endRecording();
+  view.scheduleFrame();
+}
+
+void main() {
+  
+  root = new GameWorld(view.width, view.height);
+  
+  for (int i = 0; i < 50; i++) {
+    root.addAsteroid(10.0);
+  }
+  
+  for (int i = 0; i < 50; i++) {
+      root.addAsteroid(20.0);
+    }
+  
+  view.setBeginFrameCallback(beginFrame);
+  view.scheduleFrame();
+}