Impossible to get (or even calculate) the Panelizer view mode in theme layer

Problem/Motivation

Discovered in #1889798-22: Panelizer support (use Panels' render pipeline to re-render edited field panes in Panels).

Panelizer has "panelizer view modes", similar to "entity view modes". However, they're independent of "entity view modes": you can configure them inside Panelizer, not in the Entity API UI.

Panelizer, when rendering entity field panes, then essentially calls field_view_field() with its display settings ($display = array(…)), not a view mode ($display = 'teaser'). This effectively means that in HOOK_preprocess_field(), $variables['element']['#view_mode'] === '_custom_display'. And there is no way to retrieve the panelizer view mode that is being applied, this is the problem.
At least, that's impossible AFAICT. If it is, show me how and you can close this issue right away.

Proposed resolution

My work-around for now is this:

/**
 * Implements hook_panelizer_pre_render_alter().
 *
 * Panelizer fails to communicate to the theme layer what view mode an entity's
 * fields are being rendered in, so we unfortunately have to do that ourselves.
 */
function panelizer_panelizer_pre_render_alter(&$panelizer, &$display, &$entity) {
  foreach ($entity->panelizer as $view_mode_key => $reference_panelizer) {
    if ($reference_panelizer === $panelizer) {
      $entity->panelizer_view_mode_key = $view_mode_key;
      break;
    }
  }
}

With this in place, I can get the "panelizer view mode" in HOOK_preprocess_field() via $variables['element']['#object']->panelizer_view_mode_key.

Remaining tasks

TBD

User interface changes

None.

API changes

TBD