ffmpeg: copy b frame codec delay back from decoder to demuxer

The demuxer needs this value to generate correct timestamps in some corner cases
Ideally the parser would always set this correctly, but some parsers lac support
for extracting this value, also its not trivial.
This fixes a regression
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -1856,6 +1856,7 @@ static int decode_video(InputStream *ist, AVPacket *pkt, int *got_output)
     ret = avcodec_decode_video2(ist->dec_ctx,
                                 decoded_frame, got_output, pkt);
     update_benchmark("decode_video %d.%d", ist->file_index, ist->st->index);
+    ist->st->codec->has_b_frames = ist->dec_ctx->has_b_frames;
 
     if (*got_output || ret<0 || pkt->size)
         decode_error_stat[ret<0] ++;