[BUGFIX] Use valid PHP 8 types related to GDImage in GifBuilder

Since PHP 8.0, there is a new "\GDImage" object that
is used for image manipulation functions. Before,
this was a "resource", but can now be type-hinted properly
in all cases regarding GifBuilder.

This reduces the amount of PHPstan errors for GifBuilder et al
to a minimum.

Resolves: #101967
Releases: main
Change-Id: I0653452efb6295e103c57a53bc60f4de7174b349
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81097


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Andreas Nedbal <andy@pixelde.su>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Andreas Nedbal <andy@pixelde.su>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Oliver Bartsch <bo@cedev.de>