Unreliable bindings for libpng 1.6.
They're likely to bring sorrow and regret. Please use a native Rust PNG library instead (e.g. [LodePNG 2.x](https://crates.io/crates/lodepng)).
Bundles libpng 1.6.37 as a fallback. Please [check](http://www.libpng.org/pub/png/libpng.html) whether this version is still secure before using this package.