demux_mkv: ignore DefaultDuration in some cases

This fixes playback of the sample linked by FFmpeg ticket 2508. The fix
follows ffmpeg commit 6158a3b (although it's not exactly the same).

The problem here is that the file contains an apparently non-sense
DefaultDuration value. DefaultDuration for audio tracks is used to
derive PTS values for packets with no timestamps, like they can happen
with frames inside a laced block. So the first packet of a SimpleBlock
will have a correct PTS, while the PTS values of the following packets
are calculated using DefaultDuration, and thus are broken.

This leads to seemingly ok playback, but broken A/V sync. Not using the
DefaultDuration value will leave the PTS values of these packets unset,
and the audio decoder can derive them from the output instead.

The fix more or less uses a heuristic to detect the broken case: if the
sample rate is 8 KHz (Matroska default, can assume unset), and the codec
is AC3 (as the broken file did), don't use it. I'm not sure why this
should be done only for AC3, maybe the muxing application (mkvmerge
v4.9.1) has known issues with AC3. AC3 also doesn't support 8 KHz as
sample rate natively.

(By the way, I'm not sure why we should honor the DefaultDuration at all
for audio. It doesn't seem to be needed. You can't seek to these frames,
and decoders should always be able to produce perfect PTS values by
adding the duration of the decoded audio to the first PTS.)

demux/demux_mkv.c

@@ -1587,6 +1587,12 @@ static int demux_mkv_open_audio(demuxer_t *demuxer, mkv_track_t *track)
         goto error;
     }
 
+    // Some files have broken default DefaultDuration set, which will lead to
+    // audio packets with incorrect timestamps. This follows FFmpeg commit
+    // 6158a3b, sample see FFmpeg ticket 2508.
+    if (sh_a->samplerate == 8000 && strcmp(track->codec_id, MKV_A_AC3) == 0)
+        track->default_duration = 0;
+
     mp_set_audio_codec_from_tag(sh_a);
 
     return 0;