[BUGFIX] Gifbuilder: Fix image-mask-functionality

Mask-functionality is broken since the
introduction of FAL.

Change-Id: Iff28d9561e10f7581041bcc35bd56dfc972954b3
Resolves: #59392
Releases: 6.3, 6.2, 6.1
Reviewed-on: https://review.typo3.org/31202
Reviewed-by: Markus Klein
Tested-by: Markus Klein