lavf/srtenc: add AVFMT_TS_NONSTRICT flag.

There is not really a problem in having two events at the same time.
Even if it's not perfectly correct, it helps remuxing more files
(typically our FATE sample).

libavformat/srtenc.c

@@ -92,6 +92,6 @@ AVOutputFormat ff_srt_muxer = {
     .priv_data_size = sizeof(SRTContext),
     .write_header   = srt_write_header,
     .write_packet   = srt_write_packet,
-    .flags          = AVFMT_VARIABLE_FPS,
+    .flags          = AVFMT_VARIABLE_FPS | AVFMT_TS_NONSTRICT,
     .subtitle_codec = AV_CODEC_ID_SUBRIP,
 };