This updates libpng16 with all the test changes from libpng17,
including changes to pngvalid.c to ensure that the original,
distributed, version of contrib/visupng/cexcept.h can be used.

pngvalid contains the correction to the use of SAVE/STORE_
UNKNOWN_CHUNKS; a bug revealed by changes in libpng 1.7.  More
tests contain the --strict option to detect warnings and the
pngvalid-standard test has been corrected so that it does not
turn on progresive-read (there is a separate test which does
that.)

Some signed/unsigned fixes have been made.
Signed-off-by: NJohn Bowler <jbowler@acm.org>

pngstest.c, and pngimage.c. Most seem harmless, but png-fix-itxt
would only work with iTXt chunks with length 255 or less.

variables in the Try macro needs to be volatile to prevent value
being lost over the setjmp.

in the manual, example.c, pngtest.c, and applications in the contrib directory.
It was incorrect advice.

in the code the number of elements in an array is calculated using
sizeof.  This generates a compile time constant of type (size_t) which
is then typically assigned to an (unsigned int) or (int). Some versions
of GCC on 64-bit systems warn about the apparent narrowing, even though
the same compiler does apparently generate the correct, in-range,
numeric constant.  This adds appropriate, safe, casts to make the
warnings go away.

if not already defined.

because they were handled improperly by Portland Group's PGI-14.1 - PGI-14.3
when using its "__builtin_pow()" function.

PNG_MAXIMUM_INFLATE_WINDOW option, so #define it out when appropriate in
pngvalid.c

with test programs.

pngvalid.c

this version of pngvalid in libpng 1.5

This reverts to the previous 'static' implementation and works round
the 'unused static function' warning by using PNG_UNUSED().

configurations don't build because of missing contrib/libtests test programs
and overly complex dependencies in scripts/pnglibconf.dfa. This change adds
contrib/conftest/*.dfa files that can be used in automatic build
scripts to ensure that these configurations continue to build.

it can manage, limited by the number of rows.

requirements of png_modifier are greater than that of png_store and as a
consequence compilation of pngvalid.c results in a warning about increased
alignment requirements because of the bare cast to (png_modifier*). The code
is safe, because the pointer is known to point to a stack allocated
png_modifier, but this change avoids the warning.

  Fixed default behavior of ARM_NEON_API. If the ARM NEON API option was
    compiled without the CHECK option it defaulted to on, not off.

  Check user callback behavior in pngunknown.c. Previous versions compiled
    if SAVE_UNKNOWN was not available but did nothing since the callback
    was never implemented.

  Merged pngunknown.c with 1.7 version and back ported 1.7 improvements/fixes