take over jp2k image handling

[jcupitt]

We could make the kakadu loader take over all jpeg2000 image handling for libvips, though this would probably break some stuff.

[scossu]

What are the risks? This is a nice to have, but if there is a risk of breakage (especially in edge cases that we may not catch right away) I could do away with it.

What is the default behavior of other modules (e.g. OPJ)?

[jcupitt]

The APIs for kakaduload and save don't exactly match the built-in libvips jp2k load and save, so you can't just swap them. It shouldn't crash, but you could get "arg not known" messages, or find settings unexpectedly had no effect.

It's probably not a very serious worry.