doc: add FFREPORT example

And fix a few nits.

Signed-off-by: Lou Logan <lou@lrcd.com>
avfilter/vf_uspp: assert that the qp_type is valid

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/vf_uspp: Add grayscale support

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