Temporary fix to pnm/tiff issue that breaks unpaper
Bug fix: leptonica generates .png when asked to produce .pbm/pgm/ppm

Leptonica does not interpret those extensions correctly.  However, when
asked to produce a .pnm file, it will produce the expected .pbm/pgm/ppm
file depending on the input.  So ask it to produce a .pnm and then
adjust the extension.

And add a test case.
Reverse tiff change
Merge branch 'for-upstream/leptdeskew' into develop