threshold if none is given.  Also depreciated auto_threshold_image() since
using partition_pixels() to pick the threshold is generally better.

to output from the subprocesses.

conform to dlib's generic image interface.  This makes dealing with
numpy images in pybind11 modules much nicer.

* Supporting bottom-up DIBs

* Variable renamed

a part with an automatically selected integer label if and only if you haven't
specified any string part labels and all the existing parts are already integer
labeled.  This makes it way easier to quickly add parts.

that it's going to move the matrix into the result.