contrib: png: update to 1.6.40

Changes from version 1.6.39 to version 1.6.40
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 * 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