ImageConverter: fix dimensions of the last tile

Suggested by Kristian Kjaergaard on the ome-users mailing list:

http://lists.openmicroscopy.org.uk/pipermail/ome-users/2013-September/003940.html

If the width (or height) of the image is evenly divisible by the tile
width (or height), the previous behavior was to set the width (or
height) of the last tile to be converted to 0.
OME-TIFF: prevent AIOOB exception when writing tiles to an OME-TIFF
Merge pull request #711 from melissalinkert/from-kristian-kjaergaard

ImageConverter: fix dimensions of the last tile