Can't substitute failover default view mode

The following code in a custom module also does the trick, in case anyone needs this right away. But I still think this is a bug, since you are able to substitute search_index with page_manager in the UI, so it should also work in the code.

/**
 * Implements hook_node_update_index().
 */
function mymodule_node_update_index($node) {
  // Add Panelized full page content to search index.
  if ($handler = panelizer_entity_plugin_get_handler('node')) {
    $view_mode = 'page_manager';
    if ($info = $handler->render_entity($node, $view_mode)) {
      $build['#view_mode'] = 'default';
      $build['#theme'] = 'panelizer_view_mode';
      $build['#panelizer'] = $node->panelizer[$view_mode];
      $build['#panelizer_content'] = $info;
      $build['#panelizer_handler'] = $handler;
      $build['#panelizer_entity'] = $node;
      $build['#panelizer_bundle'] = $node->type;
      $build['#panelizer_entity_id'] = $node->nid;
      return drupal_render($build);
    }
  }
}

There are several problems with search and Panelizer, but I think for now we can limit this issue to one very specific bug.

In PanelizerEntityDefault::get_view_mode(), if no view mode is found and the fallback 'default' is assigned, we should check if 'default' is configured to be substituted, since this is possible in the UI:

The attached patch fixes this.

The previous patch still wouldn't allow to substitute non-default view modes with 'page_manager', because 'page_manager' is not configured in the 'view mode status' array.

One solution is to add 'page manager' to that array in panelizer_entity_plugin_process(), which may still be a good idea.

But by simply re-arranging PanelizerEntityDefault::get_view_mode() to fallback to default first, then check for the substitute, we actually resolve both issues: 1) we can substitute when falling back to the default view mode, and 2) we can substitute any view mode with 'page_manager'.