demux_mkv: implement audio skipping/trimming

This mechanism was introduced for Opus, and allows correct skipping of
"preroll" data, as well as discarding trailing audio if the file's
length isn't a multiple of the audio frame size.

Not sure how to handle seeking. I don't understand the purpose of the
SeekPreRoll element.

This was tested with correctness_trimming_nobeeps.opus, remuxed to mka
with mkvmerge v7.2.0. It seems to be correct, although the reported file
duration is incorrect (maybe a mkvmerge issue).

1 files changed, 16 insertions, 0 deletions

diff --git a/demux/packet.c b/demux/packet.c
index 959c707732..968427dee5 100644
--- a/demux/packet.c
+++ b/demux/packet.c
@@ -20,6 +20,7 @@
 #include <assert.h>
 
 #include <libavcodec/avcodec.h>
+#include <libavutil/intreadwrite.h>
 
 #include "common/av_common.h"
 #include "common/common.h"
@@ -113,3 +114,18 @@ struct demux_packet *demux_copy_packet(struct demux_packet *dp)
     new->duration = dp->duration;
     return new;
 }
+
+int demux_packet_set_padding(struct demux_packet *dp, int start, int end)
+{
+    if (!start  && !end)
+        return 0;
+    if (!dp->avpacket)
+        return -1;
+    uint8_t *p = av_packet_new_side_data(dp->avpacket, AV_PKT_DATA_SKIP_SAMPLES, 10);
+    if (!p)
+        return -1;
+
+    AV_WL32(p + 0, start);
+    AV_WL32(p + 4, end);
+    return 0;
+}