Support modern PHP

imagelightnessat.func.php

@@ -11,7 +11,7 @@
  * by Jed Smith <?php $u = "jed"; $d = "bz"; printf("<%s@%s.%s>", $u, $u, $d) ?>
  */
 function imagelightnessat($img, $x, $y) {
-    if (!is_resource($img)) {
+    if (!$img instanceof \GdImage && !is_resource($img)) {
         trigger_error('imagelightnessat(): supplied argument is not a valid '
             . 'Image resource', E_USER_WARNING);