imgutils: make systematic palette opaque.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
vf_format: rework format list parsing

Avoid using AV_PIX_FMT_NB, since that hardcodes the number of pixel
formats into lavfi and will break when a shared lavu is updated, adding
new pixel formats.
vf_scale: use the pixfmt descriptor API

Avoid using AV_PIX_FMT_NB, since that hardcodes the number of pixel
formats into lavfi and will break when a shared lavu is updated, adding
new pixel formats.
vf_fieldorder: avoid using AV_PIX_FMT_NB

That hardcodes the number of pixel formats into lavfi and will break
when a shared lavu is updated, adding new pixel formats.
buffersrc: avoid using AV_PIX_FMT_NB

That hardcodes the number of pixel formats into lavfi and will break
when a shared lavu is updated, adding new pixel formats.
lavfi/formats: avoid using AV_{PIX,SAMPLE}_FMT_NB

Thatt hardcodes the number of formats into lavfi and will break when a
shared lavu is updated, adding new formats.