Rust bindings for libpng 1.6.

libpng is likely to bring sorrow and regret. I strongly recommend to use a native Rust PNG library instead (e.g. [LodePNG](https://lib.rs/crates/lodepng)).

This crate 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.