Fix export of multiple files

See https://trac.openmicroscopy.org.uk/ome/ticket/10635

This needs manual testing, ideally both in ImageJ and using ```bfconvert```.  All stack output formats (OME-TIFF, TIFF, QuickTime, AVI, PNG, ICS) are affected; readers are not affected at all.

In ImageJ, best would be to test opening a single dataset with SizeZ, SizeC, and SizeT all greater than 1, then exporting to each of the above formats with one or more of the "Write each ..." boxes checked.  No error message should be shown, and the converted files should all open correctly.

With ```bfconvert```, just testing that ```bfconvert input-file output-file``` works for each of the above output formats should be enough.