jp2k: Support OpenJPEG 2.1.0

OpenJPEG 2.0.1 would also work, except that it has a different signature
for opj_stream_set_user_data().  Given the current state of OpenJPEG 2.x
deployment, supporting 2.0.1 doesn't seem worth the extra configure check
and preprocessor macro.

Fixes #109.