Upgrade libpng from 1.6.39 to 1.6.51

Currently using 1.6.39 dll/3rdparty/libpng

https://git.reactos.org/?p=reactos.git;a=blob;f=dll/3rdparty/libpng/png.c#l17

https://github.com/reactos/reactos/blob/master/media/doc/3rd%20Party%20Files.txt#L36

Changes

• https://raw.githubusercontent.com/pnggroup/libpng/v1.6.50/ANNOUNCE
• https://raw.githubusercontent.com/pnggroup/libpng/v1.6.49/ANNOUNCE
• ...

Changes from version 1.6.39 to version 1.6.40

• Fixed the eXIf chunk multiplicity checks.
• Fixed a memory leak in pCAL processing.
• Corrected the validity report about tRNS inside png_get_valid().
• Fixed various build issues on *BSD, Mac and Windows.
• Updated the configurations and the scripts for continuous integration.
• Cleaned up the code, the build scripts, and the documentation.

http://www.libpng.org/pub/png/libpng.html