Afaict, our ffmpeg can decode jpeg 2000 (in both J2K and JP2 codec flavors
as well as high bitdepths), so added these extensions to the supported
list. (havent checked if the ffmpeg we build depends on
WITH_IMAGE_OPENJPEG?)
Also IMB_ispic > IMB_ispic_type > imb_is_a_jp2 was only testing for jp2,
now do both jp2/j2k.