contrib: png: update to 1.6.40
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.
See: https://sourceforge.net/p/libpng/code/ci/libpng16/tree/ANNOUNCE