PCORAW: fix offsets for files larger than 4 GB

See http://www.openmicroscopy.org/community/viewtopic.php?f=13&t=7636 and the file in ```pcoraw/alex/```.  Without this change, planes beyond 5257 will not have the timestamp and image number in the same location as in previous planes.  With this change, all ~6300 planes should have an image number and timestamp in the same place in the upper left corner.