ExporterI: set tile size for exported files to 128x128

This prevents the OME-TIFF writer from writing each row of pixels as a
separate strip, which should improve performance upon re-importing the
exported file.

See http://trac.openmicroscopy.org.uk/ome/ticket/11850.  In the test case outlined in the ticket, the import times should now be roughly similar for the original file and the exported OME-TIFF.

Whether or not we really want to use 128x128 as the tile size across the board is up for debate, but it seemed like a reasonable value to use for now.