ffmpeg: move field order decision making to encoder initialization

ffmpeg: move field order decision making to encoder initialization

We now have the possibility of getting AVFrames here, and we should
not touch the muxer's codecpar after writing the header.

Results of FATE tests change as the MXF and Matroska muxers actually
write down the field/frame coding type of a stream in their
respective headers. Before this change, these values in codecpar
would only be set after the muxer was initialized. Now, the
information is also available for encoder and muxer initialization.