ffmpeg: do not give away our reference to the frame.

Note: the logic here is temporary until the updates to
ffmpeg.c are merged.

ffmpeg.c

@@ -1737,7 +1737,8 @@ static int decode_video(InputStream *ist, AVPacket *pkt, int *got_output)
                                  AV_BUFFERSRC_FLAG_NO_COPY |
                                  AV_BUFFERSRC_FLAG_PUSH);
         } else
-        if(av_buffersrc_add_frame_flags(ist->filters[i]->filter, decoded_frame, AV_BUFFERSRC_FLAG_PUSH)<0) {
+        if(av_buffersrc_write_frame(ist->filters[i]->filter, decoded_frame)<0) {
+            /* TODO add back AV_BUFFERSRC_FLAG_PUSH */
             av_log(NULL, AV_LOG_FATAL, "Failed to inject frame into filter network\n");
             exit(1);
         }