lavf: deprecate compute_pkt_fields2

All encoders set pts and dts properly now (and have been doing that for
a while), so there is no good reason to do any timestamp guessing in the
muxer.

The newly added AVStreamInternal will be later used for storing all the
private fields currently living in AVStream.

libavformat/utils.c

@@ -2460,6 +2460,8 @@ static void free_stream(AVStream **pst)
     if (st->attached_pic.data)
         av_packet_unref(&st->attached_pic);
 
+    av_freep(&st->internal);
+
     av_dict_free(&st->metadata);
     av_freep(&st->probe_data.buf);
     av_free(st->index_entries);
@@ -2551,6 +2553,11 @@ AVStream *avformat_new_stream(AVFormatContext *s, const AVCodec *c)
         av_free(st);
         return NULL;
     }
+
+    st->internal = av_mallocz(sizeof(*st->internal));
+    if (!st->internal)
+        goto fail;
+
     if (s->iformat) {
         /* no default bitrate if decoding */
         st->codec->bit_rate = 0;
@@ -2583,6 +2590,9 @@ AVStream *avformat_new_stream(AVFormatContext *s, const AVCodec *c)
 
     s->streams[s->nb_streams++] = st;
     return st;
+fail:
+    free_stream(&st);
+    return NULL;
 }
 
 AVProgram *av_new_program(AVFormatContext *ac, int id)