Sky example game enhancements, adds preloading of images and adds transform modes to SpriteBox

sky/examples/game/main.dart

Index: sky/examples/game/main.dart
diff --git a/sky/examples/game/main.dart b/sky/examples/game/main.dart
index 4a584eb2d46037e4e07471b7902ba77efbfe3ee1..9d54b06a0a0f63bd2071e7fdc27336d880dc86bb 100644
--- a/sky/examples/game/main.dart
+++ b/sky/examples/game/main.dart
@@ -6,6 +6,15 @@ import 'package:sky/framework/app.dart';
 AppView app;
 
 void main() {
+  // Load images
+  new ImageMap([
+      "https://raw.githubusercontent.com/slembcke/GalacticGuardian.spritebuilder/GDC/Packages/SpriteBuilder%20Resources.sbpack/resources-auto/BurnTexture.png",
+      "https://raw.githubusercontent.com/slembcke/GalacticGuardian.spritebuilder/GDC/Packages/SpriteBuilder%20Resources.sbpack/Sprites/resources-auto/asteroid_big_002.png",
+    ],
+    allLoaded);
+}
+
+void allLoaded(ImageMap loader) {
   // Create a new app with the sprite box that contains our game world
-  app = new AppView(new SpriteBox(new GameWorld(1024.0, 1024.0),SpriteBoxTransformMode.letterbox));
+  app = new AppView(new SpriteBox(new GameWorld(loader),SpriteBoxTransformMode.letterbox));
 }