libpng: Update to 1.6.43

[Chubercik]

https://sourceforge.net/projects/libpng/files/libpng16/1.6.43/

Changes from version 1.6.40 to version 1.6.41:

• Added SIMD-optimized code for the Loongarch LSX hardware.
• Fixed the run-time discovery of MIPS MSA hardware.
• Fixed an off-by-one error in the function png_do_check_palette_indexes, which failed to recognize errors that might have existed in the first column of a broken palette-encoded image. This was a benign regression accidentally introduced in libpng-1.6.33. No pixel was harmed.
• Fixed, improved and modernized the contrib/pngminus programs, i.e., png2pnm.c and pnm2png.c
• Removed old and peculiar portability hacks that were meant to silence warnings issued by gcc version 7.1 alone.
• Fixed and modernized the CMake file, and raised the minimum required CMake version from 3.1 to 3.6.
• Allowed the configure script to disable the building of auxiliary tools and tests, thus catching up with the CMake file.
• Fixed a build issue on Mac.
• Moved the Autoconf macro files to scripts/autoconf.
• Moved the CMake files (except for the main CMakeLists.txt) to scripts/cmake and moved the list of their contributing authors to scripts/cmake/AUTHORS.md
• Updated the CI configurations and scripts.
• Relicensed the CI scripts to the MIT License.
• Improved the test coverage.

Changes from version 1.6.41 to version 1.6.42:

• Fixed the implementation of the macro function png_check_sig. This was an API regression, introduced in libpng-1.6.41.

Changes from version 1.6.42 to version 1.6.43:

• Fixed the row width check in png_check_IHDR(). This corrected a bug that was specific to the 16-bit platforms, and removed a spurious compiler warning from the 64-bit builds.
• Added eXIf chunk support to the push-mode reader in pngpread.c.
• Added contrib/pngexif for the benefit of the users who would like to inspect the content of eXIf chunks.
• Added contrib/conftest/basic.dfa, a basic build-time configuration.
• Fixed a preprocessor condition in pngread.c that broke build-time configurations like contrib/conftest/pngcp.dfa.
• Added CMake build support for LoongArch LSX.
• Fixed a CMake build error that occurred under a peculiar state of the dependency tree. This was a regression introduced in libpng-1.6.41.
• Marked the installed libpng headers as system headers in CMake.
• Updated the build support for RISCOS.
• Updated the makefiles to allow cross-platform builds to initialize conventional make variables like AR and ARFLAGS.
• Added various improvements to the CI scripts in areas like version consistency verification and text linting.
• Added version consistency verification to pngtest.c also.

Listed contributors have been removed to make reading of the above list(s) easier.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 4.2.2.
Cherry-picked for 4.1.4.

[akien-mga]

Cherry-picked for 3.6.
Cherry-picked for 3.5.4.