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.

(Bug report by Thuan Pham, SourceForge issue #278)

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.

Some are user facing. Some are in actual code. Most are in source comments. Also, please double check the changes in contrib/tools/pngfix.c

in attempt to stop an oss-fuzz "use of ininitialized value" issue

deflate buffer for each row (Bug report by Andrew Church).

with "II" or "MM".

to account for the minimum 'deflate' stream.