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.