avformat/avienc: fix fields-per-frame value for interlaced video streams

Writes one set of field framing information for progressive streams and
two sets for interlaced streams. Fixes ticket #6383.

Unfortunately the OpenDML v1.02 document is not very specific on what
value to use for start_line when frame data is not coming from a
capturing device, so this is just using 0/1 depending on the field order
as a best-effort guess.

Signed-off-by: Tobias Rapp <t.rapp@noa-archive.com>