demux_mkv: support V_FFV1 tag

FFmpeg encodes FFV1 with the V_MS/VFW/FOURCC tag, but GStreamer's
matroskamux uses the V_FFV1 tag, so we need to recognize it.

demux/demux_mkv.c

@@ -1447,6 +1447,7 @@ static const char *const mkv_video_tags[][2] = {
     {"V_PNG",               "png"},
     {"V_AVS2",              "avs2"},
     {"V_AVS3",              "avs3"},
+    {"V_FFV1",              "ffv1"},
     {0}
 };