avfilter/vf_uspp: fix used pix_fmt

Found-by: carl
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/vf_uspp: fix used chroma subsampling factors

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/vf_uspp: fix gray pixfmt handling

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