code: function updateBounds() {
           if (this.shapes.length > 0) {
             // reset the rect with default values
             var _bounds = this.shapes[0].getBounds();
  
             this.pos.setV(_bounds.pos);
             this.resize(_bounds.width, _bounds.height);
  
             for (var i = 1; i < this.shapes.length; i++) {
               this.union(this.shapes[i].getBounds());
             }
           } // trigger the onBodyChange
  
  
           if (typeof this.onBodyUpdate === "function") {
             this.onBodyUpdate(this);
           }
  
           return this;
         }
docs: update the body bounding rect (private)
  the body rect size is here used to cache the total bounding rect
  @private
  @name updateBounds
  @memberOf me.Body
  @function