Add MKV tags for AVS2 and AVS3 codecs.

Signed-off-by: TianBo Zheng <naturalwalker@hotmail.com>

MKV codec mappings:
  V_AVS2 and V_AVS3 (https://github.com/ietf-wg-cellar/matroska-specification/blob/master/codec_specs.md)

Encoding tool:
  Ffmpeg with AVS2/AVS3 enabled: https://github.com/xatabhk/FFmpeg-avs2-avs3/releases
  Command line:

             ffmpeg -i xxxx.mp4 -vcodec avs2 -acodec copy xxxx_avs2.mkv
             ffmpeg -i xxxx.mp4 -vcodec avs2 -speed_level 4 -acodec copy xxxx_avs2.mkv`

Players:
  (1) Ffmpeg with avs2/avs3 enabled: (https://github.com/xatabhk/FFmpeg-avs2-avs3/releases):
    Command line:

        ffplay xxxx_avs2.mkv
    ffplay xxxx_avs3.mkv
  (2) VLC 3.0.x with AVS2/AVS3 enabled: https://github.com/xatabhk/vlc-3.0-avs2-avs3/releases
  (3) Mpc-hc 1.9.x with AVS2/AVS3 enabled: https://gitee.com/zhengtianbo/cavs-avs2-avs3_decoder_added_to_mpc_hc/releases

AVS2/AVS3 MKV samples:
  https://github.com/xatabhk/avs2-avs3-video-samples

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/matroska.c

@@ -78,6 +78,8 @@ const CodecTags ff_mkv_codec_tags[]={
     {"S_HDMV/TEXTST"    , AV_CODEC_ID_HDMV_TEXT_SUBTITLE},
 
     {"V_AV1"            , AV_CODEC_ID_AV1},
+    {"V_AVS2"           , AV_CODEC_ID_AVS2},
+    {"V_AVS3"           , AV_CODEC_ID_AVS3},
     {"V_DIRAC"          , AV_CODEC_ID_DIRAC},
     {"V_FFV1"           , AV_CODEC_ID_FFV1},
     {"V_MJPEG"          , AV_CODEC_ID_MJPEG},