Make openjp2 opt-in

[Enet4]

openjpeg-sys provides a good implementation, but requires linking with non-Rust code. openjp2 is written in Rust, but only works in some platforms due to the nature of the code port. Neither of which is ready to be a default implementation.

This PR resolves #446 by requiring consumers to pick a JPEG 2000 implementation.

Summary

• remove openjp2 as a default Cargo feature
• remove Windows-specific Cargo features, since the default now works fine
• tweak CI workflow accordingly