getImageStringForLoad and use tiff for converting to imagick

maybe once libvips/php-vips-ext#13 is done, we can use that

Author: chregu

lib/Imagine/Vips/Image.php

@@ -842,6 +842,17 @@ private function prepareOutput(array $options, $path = null): self
         // FIXME: layer support, merge them if $options['animated'] != true or $options['flatten'] == true
     }
 
+    /**
+     * @param $res
+     * @return mixed
+     */
+    protected function getImageStringForLoad(\Jcupitt\Vips\Image $res)
+    {
+        //return $res->pngsave_buffer();
+        //file_put_contents("foo.tiff", $res->tiffsave_buffer(['properties' => true, 'compression' => ForeignTiffCompression::NONE]));
+        return $res->tiffsave_buffer(['compression' => ForeignTiffCompression::NONE]);
+    }
+
     /**
      * Internal.
      *