Request For Comments: Image decoding

[s3bk]

Right now Image decoding is part of this crate behind various feature gates.

• Not everyone actually wants to decode images.
• There are two C libraries used to decode Jpeg2000 and JBIG2, and both have a list of security issues.

I want to remove those libraries.
One option would be to have two statics that are used to register callbacks to decode the respective data, making the user responsible to deal with this, possibly by sandboxing the C libraries.