Support FFV1 in Matroska with codecid V_FFV1
Dave Rice provided two ffv1 samples with new codec_id V_FFV1, see https://trac.ffmpeg.org/ticket/6206
Specification is here: https://github.com/Matroska-Org/matroska-specification/blob/master/codec_specs.md#v_ffv1
It would be helpful if this could be backported to 2.2.