Use pixfmt_desc instead of codec_t for internal compression
representation. This better alignes eg. YUV 10-bit 4:4:4 which has been
deduced as Y416 and eg. for DeckLink, R12L was chosen because it was
thought to be 16 bit, not 10.
This fixes:
uv -t testcard:codec=R10k -c libavcodec:encoder=libx265:yuv -d dummy:codec=decklink
being detected internally as Y416 and configured as R12L. Now it is
internally Y444_10 and output DeckLink would be configured to R10k.
Removed also params "lavd-use-10bit", "lavd-use-codec", which were
deprecated already some time and if can be replaced by
"decoder-use-codec" if needed.
