As per the const correctness rules, top-level const-ness of data
in automatic scopes does not propagate outside of these scopes
(unlike const-ness at lower levels, such as pointers to const data).

Previously, const was used liberally, but inconsistently across the
libpng codebase. Using const wherever applicable is not incorrect.
However, _consistent_ use of const is difficult to maintain in such
conditions.

In conclusion, we shall continue to use const only where doing so is
strictly necessary:

1. If a function guarantees that it will not modify an argument
   passed by pointer, the corresponding function parameter should be
   a pointer-to-const (const T *).

2. Static data should not be modified, therefore it should be const.

Reference:
Google C++ Style Guide
https://google.github.io/styleguide/cppguide.html#Use_of_const

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.

This information is maintained by the version control system.

In v1.6.0, size_t became a required type. It should be used
consistently. To maintain backwards compatibility, png_size_t
is still maintained in deprecated form.

  in pngwrite.c, and made various other fixes to png_write_eXIf().
Eliminated png_ptr->num_exif member from pngstruct.h and added num_exif
  to arguments for png_get_eXIf() and png_set_eXIf().

compression buffer (bug report by Brian Baird).  This bug was introduced in
libpng-1.6.0.

    Malaquias)

flag definitions in pngpriv.h from 0xnnnn to 0xnnnnU and trivial changes
in png.c, pngread.c, and pngwutil.c.

only one filter is a candidate for selection. This trades off code size (added
png_setup_*_row_only() functions) for speed.

code in the PAETH decoder in pngrutil.c. Build with PNG_USE_ABS to use this.

MSVC doesn't like '-(unsigned)', so replace it by 0U-(unsigned) (in 1.6).
Signed-off-by: NJohn Bowler <jbowler@acm.org>