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

See https://www.openmicroscopy.org/community/viewtopic.php?f=13&t=7560.  The file referenced in the thread is in ```zeiss-lsm/dave/```; without this change, images in series beyond the first should look mixed up, as in the screenshots posted to the forum.  With this change, all images in all series should look reasonable (i.e. similar to the first series) when opened in ImageJ or imported into OMERO.  If in doubt, the images can also be checked with Zeiss Zen (http://www.zeiss.com/microscopy/en_us/products/microscope-software/zen-lite.html).

Due to the size of the file, if opening in ImageJ you will likely not be able to open all planes in memory at once; either choose one series at a time, or select the ```Use virtual stack``` option before opening all series.

This should only affect Zeiss LSM files larger than 4GB, and I would not expect there to be any test failures as a result of this PR.