flac demuxer: parse the WAVEFORMATEXTENSIBLE_CHANNEL_MASK tag

It is used to store the channel mask for non-standard layouts.

libavformat/flacdec.c

@@ -139,9 +139,24 @@ static int flac_read_header(AVFormatContext *s)
             }
             /* process supported blocks other than STREAMINFO */
             if (metadata_type == FLAC_METADATA_TYPE_VORBIS_COMMENT) {
+                AVDictionaryEntry *chmask;
+
                 if (ff_vorbis_comment(s, &s->metadata, buffer, metadata_size)) {
                     av_log(s, AV_LOG_WARNING, "error parsing VorbisComment metadata\n");
                 }
+
+                /* parse the channels mask if present */
+                chmask = av_dict_get(s->metadata, "WAVEFORMATEXTENSIBLE_CHANNEL_MASK", NULL, 0);
+                if (chmask) {
+                    uint64_t mask = strtol(chmask->value, NULL, 0);
+                    if (!mask || mask & ~0x3ffffULL) {
+                        av_log(s, AV_LOG_WARNING,
+                               "Invalid value of WAVEFORMATEXTENSIBLE_CHANNEL_MASK\n");
+                    } else {
+                        st->codec->channel_layout = mask;
+                        av_dict_set(&s->metadata, "WAVEFORMATEXTENSIBLE_CHANNEL_MASK", NULL, 0);
+                    }
+                }
             }
             av_freep(&buffer);
         }