avcodec/mjpegdec: fix decoding 4th plane

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/mjpegdec: convert CMYK to GBRAP

Fixes Ticket2799

This should be moved into swscale once we have a CMYK pixel format

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>