getFullScreenToggle = function() {
   var fullScreenButton = document.getElementById('gjs-fullscreen-toggle');
   if (!fullScreenButton) {
      // before canvas
      fullScreenButton = document.createElement('button');
      fullScreenButton.innerHTML = 'Fullscreen';
      fullScreenButton.id = 'gjs-fullscreen-toggle';
      var canvas = getCanvas();
      canvas.parentNode.insertBefore(fullScreenButton, canvas);
      canvas.parentNode.insertBefore(document.createElement('br'), canvas);

   }
   return fullScreenButton;
}
@returns {document.Element} the canvas dom element
@ignore