Zeiss LSM: fix offset calculationg for > 4GB files with SizeT > 1

See https://www.openmicroscopy.org/community/viewtopic.php?f=13&t=7560
Merge pull request #1210 from melissalinkert/large-lsm-offsets

Zeiss LSM: fix offset calculationg for > 4GB files with SizeT > 1