lavc: set AVSubtitle.pts if possible.

If the packet has a PTS and the corresponding time base is known,
set the pts field of the decoded subtitle structure before the
call to the decoder. The decoder is still allowed to change the
PTS if necessary.

libavcodec/utils.c

@@ -1701,6 +1701,9 @@ int avcodec_decode_subtitle2(AVCodecContext *avctx, AVSubtitle *sub,
     avctx->pkt = avpkt;
     *got_sub_ptr = 0;
     avcodec_get_subtitle_defaults(sub);
+    if (avctx->pkt_timebase.den && avpkt->pts != AV_NOPTS_VALUE)
+        sub->pts = av_rescale_q(avpkt->pts,
+                                avctx->pkt_timebase, AV_TIME_BASE_Q);
     ret = avctx->codec->decode(avctx, sub, got_sub_ptr, avpkt);
     if (*got_sub_ptr)
         avctx->frame_number++;