markSelected = function(scroller) {
    var $this = $(this),
        data = $this.data(pluginName);

    var offset = {top: 0, left: 0};
    var scrollerChain = scroller;
    while (!!scrollerChain) {
      var scrollerOffset = scrollerChain.getScrollOffset();
      offset.top += scrollerOffset.top;
      offset.left += scrollerOffset.left;
      scrollerChain = scrollerChain.next;
    }

    for (var i = data.selectables.length - 1; i >=0 ; i--) {
      var selectable = data.selectables[i];
      if (overlap(data.selectionBoxExtents, selectable, offset)) {
        if (!selectable.selected) {
          $(selectable.element).addClass(pluginName + '-selected');
          selectable.selected = true;
          $this.trigger(
              pluginName + 'selecting',
              {'selecting': selectable.element});
        }
      } else if (selectable.selected) {
        $(selectable.element).removeClass(pluginName + '-selected');
        selectable.selected = false;
        $this.trigger(
            pluginName + 'unselecting',
            {'unselecting': selectable.element});
      }
    }
  }
Update the selection status of all selectable elements', depending on
whether the selection box currently touches them or not. Triggers
'selecting' and 'unselecting' events.