Updates cache for thumbnails in the current loop.

function update_post_thumbnail_cache( $wp_query = null ) {
	if ( ! $wp_query ) {
		$wp_query = $GLOBALS['wp_query'];
	}

	if ( $wp_query->thumbnails_cached ) {
		return;
	}

	$thumb_ids = array();

	foreach ( $wp_query->posts as $post ) {
		$id = get_post_thumbnail_id( $post->ID );
		if ( $id ) {
			$thumb_ids[] = $id;
		}
	}

	if ( ! empty( $thumb_ids ) ) {
		_prime_post_caches( $thumb_ids, false, true );
	}

	$wp_query->thumbnails_cached = true;
}