[TASK] Use native PHP types in protected/internal GifBuilder methods

This change cleans up GifBuilder class
further by adding types to all internal
or protected methods.

In addition, some cleanups are done:
* Renaming of `$OFFSET` to `$offset`
* Remove superfluous checks of `is_array()`

Resolves: #102347
Releases: main
Change-Id: I7888a4240f7702e3177011847f6c63a587c40c2a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81705
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>