Currently gives None for a 16 bit greyscale image rather than the
true min and max values in the 0-65536 range. The internal
ImagingGetProjection function already supports I;16 but the
_getextrema needs to know to unpack the result.
This is Christoph Gohlke's test suite from his personal PIL package found
at http://www.lfd.uci.edu/~gohlke/pythonlibs/.
This is just to bring it in as a separate commit. Future commits will align
it with Pillow.