avcodec/mjpegdec: fix width for non chroma in rescaling

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/mjpegdec: Support pix_fmt_id==0x11222200

Fixes: 4858286_300.jpg

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