Possible Group 4 decoding with fax crate?

[marknefedov]

https://github.com/pdf-rs/fax
It looks like it can be possible to implement fax/group 4 compression.

[HeroicKatora]

A few too many unwraps on user input instead of proper error handling to be considered for a default-on option, but otherwise looks promising. For example:

https://github.com/pdf-rs/fax/blob/master/src/decoder.rs#L64

I'd gladly consider a PR with binding anyways. For example, there shouldn't be an issue if users had to opt-in (as an optional feature and at runtime) to mark it as somewhat unstable. Many issues can be quickly found with fuzzer coverage as well—see this repository for an example. Also ping this issue from time-to-time as fax seems to be under development and this judgment may well change.