[BUGFIX] FAL: Image Processing doesn't respect GFX "thumbnails_png"

The Task Interface of the File Abstraction Layer defines the
target file extension for graphcal tasks. Previously this was done
with the option "thumbnails_png" from the TYPO3_CONF_VARS.

However, checking for that parameter is not done anymore within
FAL/AbstractGraphicalTask. Thus, all .gif images are shown as .png
files, even though TYPO3 is not configured like that.

This can be seen when using GM, thumbnails_png=0, and creating
a preview image of an animated GIF. The preview will not be shown
for some images then.

The patch introduces the existing option back in to 6.x/FAL.

Releases: 6.2, 6.1, 6.0
Resolves: #50193
Change-Id: Ie539fe1aa1ab7366129a156be8ce9b05a5f8d5d2
Reviewed-on: https://review.typo3.org/22850
Reviewed-by: Anja Leichsenring
Tested-by: Marc Fell
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters