OME-TIFF: close helper readers when working with single-plane files

The reader will be left open if only a tile was read, but if the full
plane from a single plane file is read then the helper reader will be
closed.  This reduces the memory usage for datasets that consist of many
single plane files.

Fixes #12219.
Change file closing check to look for lower-right-most tile
Merge pull request #1183 from melissalinkert/12219

OME-TIFF: close helper readers when working with single-plane files