img2: Add j2k file extension for JPEG 2000

Signed-off-by: Diego Biurrun <diego@biurrun.de>
pixdesc/pixfmt: Add XYZ colorspace for XYZ 12-bit values

Signed-off-by: Diego Biurrun <diego@biurrun.de>
libopenjpeg: Add support for XYZ colorspace, found in DCINEMA frames

Signed-off-by: Diego Biurrun <diego@biurrun.de>