Use Matroska document version 2 for WebM files if possible.

Google's plugin for the Internet Explorer refuses to play
files with another document version.

Fixes ticket #3583.

libavformat/matroskaenc.c

@@ -1096,7 +1096,7 @@ static int mkv_write_header(AVFormatContext *s)
     AVIOContext *pb = s->pb;
     ebml_master ebml_header, segment_info;
     AVDictionaryEntry *tag;
-    int ret, i;
+    int ret, i, version = 2;
 
     if (!strcmp(s->oformat->name, "webm")) mkv->mode = MODE_WEBM;
     else                                   mkv->mode = MODE_MATROSKAv2;
@@ -1104,7 +1104,12 @@ static int mkv_write_header(AVFormatContext *s)
     if (s->avoid_negative_ts < 0)
         s->avoid_negative_ts = 1;
 
-    for (i = 0; i < s->nb_streams; i++)
+    if (mkv->mode != MODE_WEBM ||
+        av_dict_get(s->metadata, "stereo_mode", NULL, 0) ||
+        av_dict_get(s->metadata, "alpha_mode", NULL, 0))
+        version = 4;
+
+    for (i = 0; i < s->nb_streams; i++) {
         if (s->streams[i]->codec->codec_id == AV_CODEC_ID_ATRAC3 ||
             s->streams[i]->codec->codec_id == AV_CODEC_ID_COOK ||
             s->streams[i]->codec->codec_id == AV_CODEC_ID_RA_288 ||
@@ -1116,6 +1121,11 @@ static int mkv_write_header(AVFormatContext *s)
                    avcodec_get_name(s->streams[i]->codec->codec_id));
             return AVERROR_PATCHWELCOME;
         }
+        if (s->streams[i]->codec->codec_id == AV_CODEC_ID_OPUS ||
+            av_dict_get(s->streams[i]->metadata, "stereo_mode", NULL, 0) ||
+            av_dict_get(s->streams[i]->metadata, "alpha_mode", NULL, 0))
+            version = 4;
+    }
 
     mkv->tracks = av_mallocz(s->nb_streams * sizeof(*mkv->tracks));
     if (!mkv->tracks)
@@ -1127,7 +1137,7 @@ static int mkv_write_header(AVFormatContext *s)
     put_ebml_uint   (pb, EBML_ID_EBMLMAXIDLENGTH    ,           4);
     put_ebml_uint   (pb, EBML_ID_EBMLMAXSIZELENGTH  ,           8);
     put_ebml_string (pb, EBML_ID_DOCTYPE            , s->oformat->name);
-    put_ebml_uint   (pb, EBML_ID_DOCTYPEVERSION     ,           4);
+    put_ebml_uint   (pb, EBML_ID_DOCTYPEVERSION     ,     version);
     put_ebml_uint   (pb, EBML_ID_DOCTYPEREADVERSION ,           2);
     end_ebml_master(pb, ebml_header);