[BUGFIX] Fix cropping of transparent gifs with im6.

The usage of the ! operator to force cropping empty image
canvas is inverted between ImageMagick v4 and v6 while
GraphicsMagick accepts both syntaxes. Because im4 is
deprecated, we can always use im6 syntax.

Fixes: #19045
Releases: 6.2, 6.1, 6.0
Change-Id: Iee1260a94f2686a512ce1b1df65c384319a4c4de
Reviewed-on: https://review.typo3.org/23873
Reviewed-by: Stefan Neufeind
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters