not info_ptr.

png_handle_pCAL() (Bug report by John Regehr).

    are derived from previous versions.
Removed much of the long list of previous versions from png.h and
    libpng.3.

attempts to ignore the errors in the code aren't quite enough to
deal with the 'channel selection' encoding added to libpng 1.7; abort.

functions. They unnecessarily duplicate png_set_filler() and png_set_add_alpha()
which now work properly with 16-bit images.

input that was made in version 1.6.17beta01, to preserve legacy
behavior even though it was incorrect.  Instead, added new API
png_set_filter_16() and png_set_add_alpha_16() that set a flag to
make png_do_read_filter() interpret the filler bytes properly.

replacing the value that is illegal in the PNG spec, in both signed and
unsigned values, with 0. Illegal unsigned values (anything greater than or equal
to  0x80000000) can still pass through, but since these are not illegal
in ANSI-C (unlike 0x80000000 in the signed case) the checking that
occurs later can catch them (John Bowler).

0x8000 flag definitions on 16-bit systems. They aren't supported
yet the defs *probably* work, however it seems much safer to do this
and be advised if anyone, contrary to advice, is building libpng 1.6
on a 16-bit system. It also adds back various switch default clauses
for GCC; GCC errors out if they are not present (with an appropriately
high level of warnings).

pngrtran.c to suppress clang warnings (Bug report by Viktor Szakats).

"libpng_autotools_extra" list in autogen.sh because it was causing a
false positive for missing files (bug report by Robert C. Seacord).