demux_mkv: add V_AV1 identifier for AV1

Quickly tested by a person who had FFmpeg linked with libaom.
Seems as simple as the VP9 mappings, where there is no extradata/
initialization data off-band, and just stuff in the packets
themselves.

Do note that the AV1 video format itself at this point is still
not frozen, so what you might produce one day might not be
decodable the following day.

demux/demux_mkv.c

@@ -1384,6 +1384,7 @@ static const char *const mkv_video_tags[][2] = {
     {"V_PRORES",            "prores"},
     {"V_MPEGH/ISO/HEVC",    "hevc"},
     {"V_SNOW",              "snow"},
+    {"V_AV1",               "av1"},
     {0}
 };