[BUGFIX] Fix gifbuilder imagesize calculation when condition is used

Change-Id: Ia3adede080a9d405dad17b670a7033bc6715a847
Fixes: #39625
Releases: master, 6.2
Reviewed-on: http://review.typo3.org/13530
Reviewed-on: http://review.typo3.org/35371
Reviewed-by: Steffen Müller <typo3@t3node.com>
Tested-by: Steffen Müller <typo3@t3node.com>