fftools/ffmpeg: call av_guess_frame_rate() when opening the file

It is currently called when configuring the filter, which races with the
demuxer thread.

fftools/ffmpeg.h

@@ -334,6 +334,8 @@ typedef struct InputStream {
     AVFrame *decoded_frame;
     AVPacket *pkt;
 
+    AVRational framerate_guessed;
+
     int64_t       prev_pkt_pts;
     int64_t       start;     /* time when read started */
     /* predicted dts of the next packet read for this stream or (when there are

fftools/ffmpeg_filter.c

@@ -738,7 +738,7 @@ static int configure_input_video_filter(FilterGraph *fg, InputFilter *ifilter,
     }
 
     if (!fr.num)
-        fr = av_guess_frame_rate(input_files[ist->file_index]->ctx, ist->st, NULL);
+        fr = ist->framerate_guessed;
 
     if (ist->dec_ctx->codec_type == AVMEDIA_TYPE_SUBTITLE) {
         ret = sub2video_prepare(ist, ifilter);

fftools/ffmpeg_opt.c

@@ -1065,6 +1065,8 @@ static void add_input_streams(OptionsContext *o, AVFormatContext *ic)
             ist->top_field_first = -1;
             MATCH_PER_STREAM_OPT(top_field_first, i, ist->top_field_first, ic, st);
 
+            ist->framerate_guessed = av_guess_frame_rate(ic, st, NULL);
+
             break;
         case AVMEDIA_TYPE_AUDIO:
             ist->guess_layout_max = INT_MAX;