Fix IFD writing when the "--legacy" option is used

[melissalinkert]

See #8

With this PR, the following should work:

$ bioformats2raw CMU-1.svs CMU-1.n5
$ raw2ometiff CMU-1.n5 CMU-1-legacy.ome.tiff --legacy
$ raw2ometiff CMU-1.n5 CMU-1.ome.tiff
# with private Bio-Formats 5.9:
$ showinf -noflat CMU-1-legacy.ome.tiff -omexml -nopix
# with Bio-Formats 6.x:
$ showinf -noflat CMU-1.ome.tiff -omexml -nopix

The resolutions should match between the two output files; each should have a pyramid with 9 resolutions. There should be no validation errors when displaying the OME-XML. tiffdump should confirm that there are no SubIFD tags in the CMU-1-legacy.ome.tiff file.

/cc @douglasrennick, @emilroz, @chris-allan