Remove #pre_render and #post_render from drupal_render() as it prevents proper Twig drillability

Just because core doesn't use it doesn't mean we can remove it.
Webform and CTools both use #post_render.

In light of #2047263: Provide inline_template tag within Twig that inlines a template for usage with a render array for drillability with #post_render and #theme_wrappers support, I am for closing (won't fix) this one.

I'd like to leave this on the table, actually.

We shouldn't need #pre_render and #post_render if we can do this right (though that may be in D9). I'd like to hear what the use cases are and see if we can solve those problems in other ways - now. If we can, we should kill these to make things less complicated :)

/**
 * Implements hook_page_alter().
 */
function ctools_page_alter(&$page) {
  $page['#post_render'][] = 'ctools_page_token_processing';
}

/**
 * A theme post_render callback to allow content type plugins to use page
 * template variables which are not yet available when the content type is
 * rendered.
 */
function ctools_page_token_processing($children, $elements) {
  $tokens = ctools_set_page_token();
  if (!empty($tokens)) {
    foreach ($tokens as $token => $key) {
      list($type, $argument) = $key;
      switch ($type) {
        case 'variable':
          $tokens[$token] = isset($variables[$argument]) ? $variables[$argument] : ''; 
          break;
        case 'callback':
          if (is_string($argument) && function_exists($argument)) {
            $tokens[$token] = $argument($variables);
          }   
          if (is_array($argument) && function_exists($argument[0])) {
            $function = array_shift($argument);
            $argument = array_merge(array(&$variables), $argument);
            $tokens[$token] = call_user_func_array($function, $argument);
          }   
          break;
      }   
    }   
    $children = strtr($children, $tokens);
  }
  return $children;
}

Not sure when else ctools might be able to accomplish this...

post_render stays for now.