[libpng16] Choose to use our calculated windowBits or the zlib header setting

via the benign-errors setting.  If benign errors are allowed, then ignore
the windowBits setting in the zlib header.
[libpng16] Choose to use windowBits==15 or the zlib header setting via the

benign-errors setting.  If benign errors are allowed, then ignore
the windowBits setting in the zlib header.
Zlib-1.2.8 and earlier don't allow us to decrease the windowBits, so
undid the improvement in beta04.
[libpng16] Check for EOF in contrib/pngminus/pnm2png.c (Paul Stewart).
[libpng16] Ignore "#" delimited comments in input file to pnm2png.c.
[libpng15] Reverted patches committed to the wrong branch.
[libpng15] Check for EOF in contrib/pngminus/pnm2png.c (Paul Stewart).

Ignore "#" delimited comments in input file to pnm2png.c.