ffmpeg: make 'bits_per_raw_sample' option more useful

Currently bits_per_raw_sample is exposed as an AVCodecContext option.
The option is not very useful, because ffmpeg 1) overwrites it with
a value from the upstream codec, or 2) it resets the value whenever
the video is resampled.

This patch adds the -bits_per_raw sample option to FFmpeg, and
caches the value like is done for the -pix_fmt option.

Example usage:
ffmpeg v210.avi -pix_fmt rgb48 -bits_per_raw_sample 10 out%d.dpx

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