Adds basic touch handling to sprites and optimizes transformations

sky/examples/game/lib/node_with_size.dart

 part of sprites;
 
 abstract class NodeWithSize extends Node {
   Size size;
   Point pivot;
 
   NodeWithSize() {
     size = Size.zero;
     pivot = Point.origin;
   }
 
-  NodeWithSize.withSize(Size this.size, [Point this.pivot]);
+  NodeWithSize.withSize(Size this.size, [Point this.pivot]) {
+    if (pivot == null) pivot = Point.origin;
+  }
 
   void applyTransformForPivot(PictureRecorder canvas) {
     if (pivot.x != 0 || pivot.y != 0) {
       double pivotInPointsX = size.width * pivot.x;
       double pivotInPointsY = size.height * pivot.y;
       canvas.translate(-pivotInPointsX, -pivotInPointsY);
     }
   }
 
   bool hitTest (Point nodePoint) {
 
     double minX = -size.width * pivot.x;
     double minY = -size.height * pivot.y;
     double maxX = minX + size.width;
     double maxY = minY + size.height;
     return (nodePoint.x >= minX && nodePoint.x < maxX &&
             nodePoint.y >= minY && nodePoint.y < maxY);
   }
 }