Rust bindings to libvpx.

Allows encoding of VP9 video frames, and together with [WebM](https://lib.rs/webm), cration of WebM video files.

Supports system-wide installations of libvpx. Includes bundled FFI for a few versions of libvpx. You can also enable `generate` feature of this crate to generate FFI on the fly for a custom version of libvpx.