Changes:
* Fix panic on certain crafted inputs (CVE-2023-42447)
  See also https://github.com/whisperfish/blurhash-rs/security/advisories/GHSA-cxvp-82cq-57h2
* Integrate with `image` and `gdk-pixbuf` through feature flags
* Multiple optimizations in decoding and encoding
* Add a bunch of integration tests and unit tests
* Allow decoding into a pre-existing buffer