#pre-render function arguments have changed

I just discovered this tonight the hard way. ;) In addition to _submit and _validate functions' arguments having changed, pre_render's functions' have as well.

What used to be x_prerender($form_id, $form) should now be just x_prerender($form) (or $elements, if you want to use the same nomenclature as drupal_render()).

I made an attempt at documenting this at http://drupal.org/node/144132#pre-render, so at least there's something to link to about it.