BUG: Use tolerant float compare when choosing better otsu threshold

The last bit of the double precision variance between threshold is
likely numeric noise and not consistent. The improvement should be
better than this potential error to improve consistent results.

Change-Id: I5dbefb32d53a4cb17fe215f0cce437c492a00c2e
Merge topic 'OtsuTolerance'

defdd992 BUG: Use tolerant float compare when choosing better otsu threshold