lafv/matroska: add A_OPUS/EXPERIMENTAL codec name.

The exact packing of Opus inside Matroska is not finalized.
Use A_OPUS/EXPERIMENTAL as codec name, like mkvtoolnix.
The A_OPUS name stays to let ffmpeg open files it has produced
until now, but newly produced file use the EXPERIMENTAL version.
Once the spec is stabilized it will be possible to consider
options to ensure compatibility with these files.

libavformat/matroska.c

@@ -32,6 +32,7 @@ const CodecTags ff_mkv_codec_tags[]={
     {"A_MPEG/L2"        , AV_CODEC_ID_MP2},
     {"A_MPEG/L1"        , AV_CODEC_ID_MP2},
     {"A_MPEG/L3"        , AV_CODEC_ID_MP3},
+    {"A_OPUS/EXPERIMENTAL",AV_CODEC_ID_OPUS},
     {"A_OPUS",            AV_CODEC_ID_OPUS},
     {"A_PCM/FLOAT/IEEE" , AV_CODEC_ID_PCM_F32LE},
     {"A_PCM/FLOAT/IEEE" , AV_CODEC_ID_PCM_F64LE},