Adds basic support for images in game example

sky/examples/game/lib/sprite_box.dart

 part of sprites;
 
 enum SpriteBoxTransformMode {
   nativePoints,
   letterbox,
   stretch,
   scaleToFit,
   fixedWidth,
   fixedHeight,
 }
 
 class SpriteBox extends RenderBox {
 
   // Root node for drawing
   TransformNode _rootNode;
 
   // Tracking of frame rate and updates
   double _lastTimeStamp;
   int _numFrames = 0;
 
   SpriteBoxTransformMode transformMode;
-  double systemWidth;
-  double systemHeight;
+  double _systemWidth;
+  double _systemHeight;
 
   SpriteBox(TransformNode rootNode, [SpriteBoxTransformMode mode = SpriteBoxTransformMode.nativePoints, double width=1024.0, double height=1024.0]) {
     // Setup root node
     _rootNode = rootNode;
 
     // Setup transform mode
     transformMode = mode;
-    systemWidth = width;
-    systemHeight = height;
+    _systemWidth = width;
+    _systemHeight = height;
 
     _scheduleTick();
   }
 
+  double get systemWidth => _systemWidth;
+  double get systemHeight => _systemHeight;
+
   void performLayout() {
     size = constraints.constrain(new Size.infinite());
   }
 
   void handlePointer(PointerEvent event) {
     switch (event.type) {
       case 'pointerdown':
         print("pointerdown");
         break;
     }
   }
 
   void paint(RenderNodeDisplayList canvas) {
     // Move to correct coordinate space before drawing
     double scaleX = 1.0;
     double scaleY = 1.0;
     double offsetX = 0.0;
     double offsetY = 0.0;
 
     switch(transformMode) {
       case SpriteBoxTransformMode.stretch:
-        scaleX = size.width/systemWidth;
-        scaleY = size.height/systemHeight;
+        scaleX = size.width/_systemWidth;
+        scaleY = size.height/_systemHeight;
         break;
       case SpriteBoxTransformMode.letterbox:
-        scaleX = size.width/systemWidth;
-        scaleY = size.height/systemHeight;
+        scaleX = size.width/_systemWidth;
+        scaleY = size.height/_systemHeight;
         if (scaleX > scaleY) {
           scaleY = scaleX;
-          offsetY = (size.height - scaleY * systemHeight)/2.0;
+          offsetY = (size.height - scaleY * _systemHeight)/2.0;
         }
         else {
           scaleX = scaleY;
-          offsetX = (size.width - scaleX * systemWidth)/2.0;
+          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 {
+          scaleX = scaleY;
+          offsetX = (size.width - scaleX * _systemWidth)/2.0;
+        }
+        break;
+      case SpriteBoxTransformMode.fixedWidth:
+        scaleX = size.width/_systemWidth;
+        scaleY = scaleX;
+        _systemHeight = size.height/scaleX;
+        print("systemHeight: $_systemHeight");
+        break;
+      case SpriteBoxTransformMode.fixedHeight:
+        scaleY = size.height/_systemHeight;
+        scaleX = scaleY;
+        _systemWidth = size.width/scaleY;
+        break;
       case SpriteBoxTransformMode.nativePoints:
         break;
       default:
         assert(false);
         break;
     }
 
     canvas.save();
 
     canvas.translate(offsetX, offsetY);
@@ skipping 21 matching lines @@
     // 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}");
 
     _rootNode.update(delta);
     _scheduleTick();
   }
 }