libpng: upgrade 1.6.35 -> 1.6.36

libpng: upgrade 1.6.35 -> 1.6.36

For changes, see:

https://sourceforge.net/p/libpng/code/ci/libpng16/tree/CHANGES

License-Update: Added authors to license, formatting, version changes,
export classification clarification and a new libpng2 license with
clarification:

The new libpng license comprises the terms and conditions from the zlib
license, and the disclaimer from the Boost license. The legacy libpng
license license, used until libpng-1.6.35, is appended to the
new license, following the precedent established in the Python Software
Foundation License version 2.

Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>