[BUGFIX] Try SVG first when detecting image size

By processing SVG files first and falling back to IM/GM identify, output
of a 'invalid image content' error for SVG files is avoided.

Change-Id: I44a7d822b100bdc83f7afa8aa544b6f3eff194d0
Resolves: #84586
Releases: master, 8.7
Reviewed-on: https://review.typo3.org/56521


Reviewed-by: Stephan Großberndt <stephan.grossberndt@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Tested-by: Frans Saris <franssaris@gmail.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>