Back ported from libpng 1.7
Signed-off-by: NJohn Bowler <jbowler@acm.org>

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.

This update incorporates errors resulting from the more extensive image test
suite used in libpng 1.7
Signed-off-by: NJohn Bowler <jbowler@acm.org>

Splitting the machine generated error structs out to a file allows the values to
be updated without changing pngstest.c itself, since libpng 1.6 and 1.7 have
slightly different error limits this simplifies maintenance.
Signed-off-by: NJohn Bowler <jbowler@acm.org>

This is a patch for libpng16.
It incorporates fixes to the low-bit-depth gray background handling from
libpng17 and turns one of the low-bit-depth gray cases that does work (and,
indeed, did work before) in libpng16
Signed-off-by: NJohn Bowler <jbowler@acm.org>

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>

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

  Removed some useless typecasts from contrib/tools/png-fix-itxt.c

handling in contrib/libtests/pngstest.c; possible overflow of
unsigned char in contrib/tools/png-fix-itxt.c). To use the "secure"
file handling, define PNG_USE_MKSTEMP, otherwise "tmpfile()" will
continue to be used.

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.

contrib/tools/pngfix.c to avoid confusion with the libpng private macros.

(x + (x>>16) + 32769) works as well as the previous form and avoids one shift.

 simpleover. Also added a comment to png.h pointing out that the pointer must
 be set to NULL!  (simpleover crashes with any slightly complex command lines
 without this fix.)

alpha compositing of multiple images, using the "simplified API"

png_read_row to skip the unnecessary row de-interlace stuff.