mjpegdec: support more pix_fmt_ids for grayscale

samples created with

"cjpeg -grayscale -sample 1x3 / 1x4 / 3x1 / 3x3 / 4x1 / 4x3 / 4x4"

http://www.datafilehost.com/download-d828329d.html



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