avformat/matroskaenc: wrap V_QUICKTIME codec private in something that looks...

avformat/matroskaenc: wrap V_QUICKTIME codec private in something that looks like its part of quicktime stsd

This is needed for matroska spec compliance
Fixes playback of SVQ3 in matroska with vlc
Fixes Ticket 3256
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/matroskaenc.c

@@ -548,8 +548,18 @@ static int mkv_write_codecprivate(AVFormatContext *s, AVIOContext *pb, AVCodecCo
         if (qt_id) {
             if (!codec->codec_tag)
                 codec->codec_tag = ff_codec_get_tag(ff_codec_movvideo_tags, codec->codec_id);
-            if (codec->extradata_size)
+            if (codec->extradata_size) {
+                if (   ff_codec_get_id(ff_codec_movvideo_tags, codec->codec_tag) == codec->codec_id
+                    && ff_codec_get_id(ff_codec_movvideo_tags, AV_RL32(codec->extradata+4)) != codec->codec_id
+                ) {
+                    int i;
+                    avio_wb32(dyn_cp, 0x5a + codec->extradata_size);
+                    avio_wl32(dyn_cp, codec->codec_tag);
+                    for(i=0; i<0x5a-8; i++)
+                        avio_w8(dyn_cp, 0);
+                }
                 avio_write(dyn_cp, codec->extradata, codec->extradata_size);
+            }
         } else {
             if (!ff_codec_get_tag(ff_codec_bmp_tags, codec->codec_id))
                 av_log(s, AV_LOG_WARNING, "codec %s is not supported by this format\n",