Imaris HDF: make the optimal tile size be more tile-shaped

...instead of being full-width strips.  The tile access time is not
really any different, but in most cases this will result in faster image
loading times.

See ticket #10272.
Imaris HDF: fix retrieval of tiles from lower right quadrant

NetCDF sometimes returns incorrect values when the (x, y) coordinate is
in the lower right quadrant of the image, so we correct for this by
adjusting the x coordinate and width.
Imaris HDF: fix one more set of off-by-one bugs
Imaris HDF: add some comments explaining tile retrieval
Merge pull request #384 from melissalinkert/10272

Imaris HDF: make the optimal tile size be more tile-shaped