fixes resize event stutter on iOS devices

[taberrr]

The window.onResize event was causing text stuttering/flickering when scrolling the page on iOS devices, so this PR ensures the window's width has actually changed before acting on the resize event.

[jjenzz]

@taberrr, thanks for this fix. It would be great if you could cache the window reference and add a comment explaining why this is required though please if you don't mind?

if (base.opts.responsive) {
  var $window = $(window);
  var windowWidth = $window.width();

  var resize = function() {
    // Scrolling on iOS6+ fires the resize event. This ensures the width has actually changed
    // before continuing. https://tinyurl.com/ios-scroll-resize
    if ($window.width() === windowWidth) {
      return;
    }
    
    windowWidth = $window.width();
    //etc.
  }

  $window.on('resize.' + namespace, resize);
}