PHP 8: Add support for GdImage resource objects
PHP 8: Add support for GdImage resource objects

In [PHP 8.0, `gd` resources are changed to `\GdImage` class objects](https://php.watch/versions/8.0/gdimage). This changes the `\Intervention\Image\AbstractDecoder::isGdResource` method to accept `\GdImage` objects as well as `gd` resources.

Intervention/image