Write bitstream without chroma when encoding with --input-format=P400.
This reduces bitstream size by 0-1 %, compared to coding monochrome in
420 format, and speeds up encoding slightly due to not processing
chroma.
This moves the interlacing from CLI code to api->encoder_encode, in
order to make it possible to use field coding through the lib API.
The field order is now determined per frame, as FFmpeg gives it per
frame and it's signaled per frame.
As a side effect, the CLI also now prints info from frames instead of
fields. While we might want to extend the API in the future to allow
printing of more detailed information about fields, for now it's
more important that the CLI uses the real lib API.
PSNR calculation for interlaced frames disabled until we have a way to
avoid deinterlacing the frame when it's not necessary.
