avcodec/utils: add 16bit planar YUV formats to avcodec_align_dimensions2()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/mjpegdec: Support >8bpc color mjpeg

Fixes Ticket2939

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