[BUGFIX] Don't prepend absolute image URLs

The ImageService->getImageUri() method always prepends the public URL of
the image object with a prefix, regardless whether the object already
returns a fully qualified URL. This can be the case when the FAL driver
is connecting a remote image storage that doesn't allow local processing
and requires the images to be loaded from the remote server.

Change-Id: Iac65bc93bc0d1c1cbeb2335b7e13fc2126b728b6
Resolves: #61617
Releases: 6.2, 6.3
Reviewed-on: http://review.typo3.org/32799
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>