Standard interlace handling:
* If we have interlace-mode=interleaved and the field order, we just
  set it when creating the session
* If we have interlace-mode=(interleaved|mixed) and no field order, we
  set the field order on the first buffer

The encoder session does not support changing the FieldDetail after it
has started encoding frames, so we cannot support mixed streams
correctly.

Part-of: <gstreamer/gstreamer!1214>