avformat/matroskaenc: Allow to use custom reformatting functions

Matroska uses variable-length elements and in order not to waste
bytes on length fields, the length of the data to write needs to
be known before writing the length field. Annex B H.264/5 and
WavPack need to be reformatted to know this length and this
currently involves writing the data into temporary buffers;
AV1 sometimes suffers from this as well.

This commit aims to solve this by adding a callback that is called
twice per packet: Once to get the size and once to actually write
the data. In case of WavPack and AV1 (where parsing is cheap due
to length fields) both calls will just parse the data with only
the second function writing anything. For H.264/5, the position
of the NALUs will need to be stored to be written lateron.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/matroskaenc.c

@@ -158,6 +158,8 @@ typedef struct mkv_cues {
     int             num_entries;
 } mkv_cues;
 
+struct MatroskaMuxContext;
+
 typedef struct mkv_track {
     int             write_dts;
     int             has_cue;
@@ -171,6 +173,13 @@ typedef struct mkv_track {
     int64_t         duration_offset;
     int64_t         codecpriv_offset;
     int64_t         ts_offset;
+    /* This callback will be called twice: First with a NULL AVIOContext
+     * to return the size of the (Simple)Block's data via size
+     * and a second time with the AVIOContext set when the data
+     * shall be written.
+     * The callback shall not return an error on the second call. */
+    int             (*reformat)(struct MatroskaMuxContext *, AVIOContext *,
+                                const AVPacket *, int *size);
 } mkv_track;
 
 typedef struct MatroskaMuxContext {
@@ -2433,6 +2442,8 @@ static int mkv_write_block(AVFormatContext *s, AVIOContext *pb,
 #endif
            if (par->codec_id == AV_CODEC_ID_AV1) {
         err = ff_av1_filter_obus_buf(pkt->data, &data, &size, &offset);
+    } else if (track->reformat) {
+        err = track->reformat(mkv, NULL, pkt, &size);
     } else
         data = pkt->data;
 
@@ -2483,9 +2494,13 @@ static int mkv_write_block(AVFormatContext *s, AVIOContext *pb,
     put_ebml_num(pb, track_number, track->track_num_size);
     avio_wb16(pb, ts - mkv->cluster_pts);
     avio_w8(pb, (blockid == MATROSKA_ID_SIMPLEBLOCK && keyframe) ? (1 << 7) : 0);
+    if (track->reformat) {
+        track->reformat(mkv, pb, pkt, &size);
+    } else {
     avio_write(pb, data + offset, size);
     if (data != pkt->data)
         av_free(data);
+    }
 
     if (blockid == MATROSKA_ID_BLOCK && !keyframe)
         put_ebml_sint(pb, MATROSKA_ID_BLOCKREFERENCE, track->last_timestamp - ts);