Make the old code compatible with modern PHP

lob2011/imagelightnessat.func.php

@@ -13,7 +13,7 @@ declare(strict_types=1);
  * 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);