avfilter/vf_psnr: >8 bit planar support

Signed-off-by: Paul B Mahol <onemda@gmail.com>
lavc/utils: Add support for discarding samples from the end

Adding support for discarding samples from the end based on the value in
AV_PKT_DATA_SKIP_SAMPLES side data's bytes 5-8.

Signed-off By: Vignesh Venkatasubramanian <vigneshv@google.com>

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