Style plugins can override the rendering of a single group

In order to override how a single view of a grouped view was rendered a style plugin
had to do hard work, just to, for example pass in an additional parameter to the theme function.

Before

class YourStylePlugin extends StylePluginBase {

  /**
   * Overrides \StylePluginBase::render_grouping_sets().
   */
  protected function render_grouping_sets($sets, $level = 0) {
    $output = array();
    $theme_functions = views_theme_functions('views_view_grouping', $this->view, $this->view->display_handler->display);
    foreach ($sets as $set) {
      $row = reset($set['rows']);
      // Render as a grouping set.
      if (is_array($row) && isset($row['group'])) {
        $output[] = array(
          '#theme' => $theme_functions,
          '#view' => $this->view,
          '#grouping' => $this->options['grouping'][$level],
          '#grouping_level' => $level,
          '#rows' => $set['rows'],
          '#title' => $set['group'],
        );
      }
      // Render as a record set.
      else {
        if ($this->usesRowPlugin()) {
          foreach ($set['rows'] as $index => $row) {
            $this->view->row_index = $index;
            $render = $this->view->rowPlugin->render($row);
            // Row render arrays cannot be contained by style render arrays.
            $set['rows'][$index] = drupal_render($render);
          }
        }

        $output[] = array(
          '#theme' => $this->themeFunctions(),
          '#view' => $this->view,
          '#options' => $this->options,
          '#grouping_level' => $level,
          '#rows' => $set['rows'],
          '#title' => $set['group'],
          //Additional parameter to my theme function.
          '#additional_parameter' => 'foo',
        ); 
     }
    }
    unset($this->view->row_index);
    return $output;
  }
}

After

Now all you need is to override the renderSingleGroup():

class YourStylePlugin extends StylePluginBase {

  /**
   * Overrides \StylePluginBase::renderSingleGroup().
   */
  protected function renderSingleGroup(array $rows = array()) {
    return array(
      '#theme' => $this->themeFunctions(),
      '#view' => $this->view,
      '#rows' => $rows,
      //Additional parameter to my theme function.
      '#additional_parameter' => 'foo',
    );
  }
}