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 https://trac.openmicroscopy.org.uk/ome/ticket/12219.  To verify, import the dataset in ```/ome/team/ejrozbicki/Series1``` and verify that import succeeds with standard memory settings (no additional configuration should be necessary).