nvenc doesn't support P016, but we have two problems today:
1) We declare support for YUV444P16 which nvenc also doesn't support.
We do this because it's the only pix_fmt we have that can
approximate nvenc's internal format that is YUV444P10 with data in
MSBs instead of LSBs. Because the declared format is a 16bit one,
it will be preferrentially chosen when encoding >10bit content,
but that content will normally be YUV420P12 or P016 which should
get mapped to P010 and not YUV444P10.
2) Transcoding P016 content with nvenc should be possible in a pure
hardware pipeline, and that can't be done if nvenc doesn't say it
accepts P016. By mapping it to P010, we can use it, albeit with
truncation. I have established that swscale doesn't know how to
dither to 10bits so we'd get truncation anyway, even if we tried
to do this 'properly'.