PRE-RELEASE: Complete rewrite v2

As I have been working on the Rust port I have learned the WebP and the GdkPixbuf APIs better and I decided to overhaul the C implementation while I still ponder if we should move to Rust yet. This is basically a port to C of the newer Rust implementation, I have been extra careful with memory leaks and have covered Pixbuf ownership a lot better according to documentation and other implementations.

This release fixes #61 and possibly others.

This revision fixes decoder regression noticed in #65