video: fix timeline with some container formats

Using edl or --merge-files with .avi files didn't work, because the DTS
was not offset. Only the PTS was adjusted, which led to nonsense
timestamps.

player/video.c

@@ -380,6 +380,8 @@ static int decode_image(struct MPContext *mpctx)
         return VD_WAIT;
     if (pkt && pkt->pts != MP_NOPTS_VALUE)
         pkt->pts += mpctx->video_offset;
+    if (pkt && pkt->dts != MP_NOPTS_VALUE)
+        pkt->dts += mpctx->video_offset;
     if ((pkt && pkt->pts >= mpctx->hrseek_pts - .005) ||
         d_video->has_broken_packet_pts ||
         !mpctx->opts->hr_seek_framedrop)