avfilter/vf_histeq: remove unused item from filter private context

Signed-off-by: Paul B Mahol <onemda@gmail.com>
avcodec/snowenc: fix constness of the AVFrame argument in encode_frame()

Some fields of the frame are changed to reflect encoder decissions like
if its a keyframe. It thus cannot be constant.

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