Adds basic sprite sheet support to sprites

sky/examples/game/lib/sprite_box.dart

 part of sprites;
 
 /// Options for setting up a [SpriteBox].
 ///
 ///  * [nativePoints], use the same points as the parent [Widget].
 ///  * [letterbox], use the size of the root node for the coordinate system, constrain the aspect ratio and trim off
 ///  areas that end up outside the screen.
 ///  * [stretch], use the size of the root node for the coordinate system, scale it to fit the size of the box.
 ///  * [scaleToFit], similar to the letterbox option, but instead of trimming areas the sprite system will be scaled
 ///  down to fit the box.
@@ skipping 187 matching lines @@
       case SpriteBoxTransformMode.stretch:
         scaleX = size.width/systemWidth;
         scaleY = size.height/systemHeight;
         break;
       case SpriteBoxTransformMode.letterbox:
         scaleX = size.width/systemWidth;
         scaleY = size.height/systemHeight;
         if (scaleX > scaleY) {
           scaleY = scaleX;
           offsetY = (size.height - scaleY * systemHeight)/2.0;
-        }
-        else {
+        } else {
           scaleX = scaleY;
           offsetX = (size.width - scaleX * systemWidth)/2.0;
         }
         break;
       case SpriteBoxTransformMode.scaleToFit:
         scaleX = size.width/systemWidth;
         scaleY = size.height/systemHeight;
         if (scaleX < scaleY) {
           scaleY = scaleX;
           offsetY = (size.height - scaleY * systemHeight)/2.0;
-        }
-        else {
+        } else {
           scaleX = scaleY;
           offsetX = (size.width - scaleX * systemWidth)/2.0;
         }
         break;
       case SpriteBoxTransformMode.fixedWidth:
         scaleX = size.width/systemWidth;
         scaleY = scaleX;
         systemHeight = size.height/scaleX;
         rootNode.size = new Size(systemWidth, systemHeight);
         break;
@@ skipping 142 matching lines @@
   ///     if (event.pointer == firstPointerId) {
   ///       // Do something
   ///     }
   final int pointer;
 
   /// Creates a new SpriteBoxEvent, typically this is done internally inside the SpriteBox.
   ///
   ///     var event = new SpriteBoxEvent(new Point(50.0, 50.0), 'pointerdown', 0);
   SpriteBoxEvent(this.boxPosition, this.type, this.pointer);
 }