In v1.6.0, compiler support for const became a requirement.
It should be used consistently. To maintain backwards compatibility,
PNG_CONST is still maintained in deprecated form.

buffer for multiple rows in contrib/gregbook, contrib/pngminus, example.c, and
in the manual (suggested by Jaeseung Choi).

fixed potential leak of png_pixels in contrib/pngminus/pnm2png.c

because usleep() is deprecated.

/readpng2.c from the previous update.

compiler complaints about unused pointers.

instead of the default.

ignore all unknown chunks and all known chunks except for IHDR, PLTE, tRNS,
IDAT, and IEND.  Previously it only meant ignore all unknown chunks, the
same as num_chunks == 0. Revised png_image_skip_unused_chunks() to
provide a list of chunks to be processed instead of a list of chunks to
ignore.  Revised contrib/gregbook/readpng2.c accordingly.