avformat/mpegts: handle AC-4 descriptor in DVB extension

as defined in ETSI EN 300 468 (Table 109).

This allows ffprobe to recognize that .ts
file has an ac4 stream.

Checked on the files downloaded from
https://ott.dolby.com/OnDelKits/AC-4/Download_v15.html

libavformat/mpegts.c

@@ -2320,6 +2320,10 @@ int ff_parse_mpeg2_descriptor(AVFormatContext *fc, AVStream *st, int stream_type
             }
             break;
         }
+        if (ext_desc_tag == AC4_DESCRIPTOR) {
+            st->codecpar->codec_id = AV_CODEC_ID_AC4;
+            st->codecpar->codec_type = AVMEDIA_TYPE_AUDIO;
+        }
         break;
     case AC3_DESCRIPTOR:
     case ENHANCED_AC3_DESCRIPTOR:

libavformat/mpegts.h

@@ -228,6 +228,7 @@ https://developer.apple.com/library/archive/documentation/AudioVideo/Conceptual/
 /* DVB descriptor_tag_extension values from
    ETSI EN 300 468 Table 109: Possible locations of extended descriptors */
 #define SUPPLEMENTARY_AUDIO_DESCRIPTOR 0x06
+#define AC4_DESCRIPTOR 0x15
 
 /** see "Dolby Vision Streams Within the MPEG-2 Transport Stream Format"
 https://professional.dolby.com/siteassets/content-creation/dolby-vision-for-content-creators/dolby-vision-bitstreams-in-mpeg-2-transport-stream-multiplex-v1.2.pdf */