GdImage class objects replace GD

Implementing suggestions for:
#1027 (comment)
As explained in:
https://php.watch/versions/8.0/gdimage

src/Mike42/Escpos/GdEscposImage.php

@@ -61,7 +61,7 @@ protected function loadImageData(string $filename = null)
      */
     public function readImageFromGdResource($im)
     {
-        if (!is_resource($im) && !is_object($im)) {
+        if (!is_resource($im) && !$im instanceof \GdImage) {
             throw new Exception("Failed to load image.");
         } elseif (!EscposImage::isGdLoaded()) {
             throw new Exception(__FUNCTION__ . " requires 'gd' extension.");