Update to the latest libpng 1.6 (stable) release.
Regenerate pnglibconf.h and pngprefix.h files as explained in
docs/contributing/how-to-update-third-party-library.md of the main
repository.
These files are used to prefix all public libpng symbols with "wx_" to
avoid conflicts with any other libpng instances that could be used by
the application using wxWidgets.
See fff5f7d5d83fbd20638f119237b699e38a73c539 in the main repository.
