Enable AVIF support in GD for PHP 8.1

tests/mods/modules/gd/gd-avif.php

@@ -0,0 +1,23 @@
+<?php
+/**
+ * This page should print 'OK' if everything works,
+ * 'FAIL' or nothing if an error occured.
+ */
+ini_set('display_startup_errors', 1);
+ini_set('display_errors', 1);
+error_reporting(-1);
+
+
+// Only available since PHP 8.1.x
+if (version_compare($PHP_VERSION, '8.1.0', '<=')) {
+	echo 'SKIP';
+	exit(0);
+}
+
+// Check for support
+if (!function_exists('imageavif')) {
+	echo 'FAIL: imageavif()';
+	exit(1);
+}
+
+echo 'OK';