Restore the 'allowed_themes' filter.

This filter has wrapped the network-wide themes, outside of the network-wide
allowed function (dating back to MU), hence the unusual placement. This restores
previous behavior; we will introduce new filters to enable greater flexibility.

see #21099.
for trunk.



git-svn-id: http://core.svn.wordpress.org/trunk@21686 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-theme.php

@@ -1071,7 +1071,8 @@ final class WP_Theme implements ArrayAccess {
 	 * @return array Array of stylesheet names.
 	 */
 	public static function get_allowed( $blog_id = null ) {
-		return self::get_allowed_on_network() + self::get_allowed_on_site( $blog_id );
+		$network = (array) apply_filters( 'allowed_themes', self::get_allowed_on_network() );
+		return $network + self::get_allowed_on_site( $blog_id );
 	}
 
 	/**