ffmpeg: Use the correct (dts) variables for stream copy.

Strongly based on Anton Khirnovs code.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -2297,18 +2297,18 @@ static int output_packet(InputStream *ist,
     /* handle stream copy */
     if (!ist->decoding_needed) {
         rate_emu_sleep(ist);
-        ist->pts = ist->next_pts;
+        ist->dts = ist->next_dts;
         switch (ist->st->codec->codec_type) {
         case AVMEDIA_TYPE_AUDIO:
-            ist->next_pts += ((int64_t)AV_TIME_BASE * ist->st->codec->frame_size) /
+            ist->next_dts += ((int64_t)AV_TIME_BASE * ist->st->codec->frame_size) /
                              ist->st->codec->sample_rate;
             break;
         case AVMEDIA_TYPE_VIDEO:
             if (pkt->duration) {
-                ist->next_pts += av_rescale_q(pkt->duration, ist->st->time_base, AV_TIME_BASE_Q);
+                ist->next_dts += av_rescale_q(pkt->duration, ist->st->time_base, AV_TIME_BASE_Q);
             } else if(ist->st->codec->time_base.num != 0) {
                 int ticks= ist->st->parser ? ist->st->parser->repeat_pict + 1 : ist->st->codec->ticks_per_frame;
-                ist->next_pts += ((int64_t)AV_TIME_BASE *
+                ist->next_dts += ((int64_t)AV_TIME_BASE *
                                   ist->st->codec->time_base.num * ticks) /
                                   ist->st->codec->time_base.den;
             }