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.

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

"if (x !== 0)"

PNG_GAMMA_MAC_18 as parameters for png_set_gamma().

message if the version field is not set.

a consistent style for (sizeof type) and (sizeof (array))

comments.  It has apparently been wrong since libpng-1.0.1c (confusing
use of a 1D and a 2D array for the image source).

instead of directly assigning string constants to png_textp members.
This avoids compiler warnings when -Wwrite-strings is enabled.

those uses of png_memcpy that were doing a simple assignment to assignments
(all those cases where the thing being copied is a non-array C L-value.)
Added some error checking to png_set_*() routines and removed the
reference to the non-exported function png_memcpy() from example.c. Fixed
the Visual C 64-bit build - it requires jmp_buf to be aligned, but it had
become misaligned.

Added new "png_structrp" typedef. Because of the
way libpng works both png_info and png_struct are always accessed via a
single pointer.  This means adding C99 'restrict' to the pointer gives
the compiler some opportunity to optimize the code.  This change allows that.