Revert "lavf/mp3dec: don't adjust start time; packets are not adjusted."

This causes regressions in end to end timestamps with mp3s and ffmpeg.
The revert is to avoid this regression in the 4.3 release

See: [FFmpeg-devel] [PATCH] Don't adjust start time for MP3 files; packets are not adjusted.

This reverts commit 460132c9.

libavformat/mp3dec.c

@@ -260,6 +260,10 @@ static void mp3_parse_info_tag(AVFormatContext *s, AVStream *st,
             st->first_discard_sample = -mp3->end_pad + 528 + 1 + mp3->frames * (int64_t)spf;
             st->last_discard_sample = mp3->frames * (int64_t)spf;
         }
+        if (!st->start_time)
+            st->start_time = av_rescale_q(st->start_skip_samples,
+                                            (AVRational){1, c->sample_rate},
+                                            st->time_base);
         av_log(s, AV_LOG_DEBUG, "pad %d %d\n", mp3->start_pad, mp3->  end_pad);
     }
 

tests/ref/fate/gapless-mp3

-ec876434ed65e338e07234e54d136caf *tests/data/fate/gapless-mp3.out-1
+44b42cc3a898b45507d856d0813f4f26 *tests/data/fate/gapless-mp3.out-1
 c96c3ae7bd3300fd2f4debac222de5b7
 ec876434ed65e338e07234e54d136caf *tests/data/fate/gapless-mp3.out-2
 c96c3ae7bd3300fd2f4debac222de5b7