ffmpeg: Use filter graph output frame rate also for frame duration estimation

Previously the duration was sometimes wrong, this addition
limits the value and improves which frames are choosen when
reducing the frame rate
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -895,6 +895,7 @@ static void do_video_out(AVFormatContext *s,
     double duration = 0;
     int frame_size = 0;
     InputStream *ist = NULL;
+    AVFilterContext *filter = ost->filter->filter;
 
     if (ost->source_index >= 0)
         ist = input_streams[ost->source_index];
@@ -902,6 +903,13 @@ static void do_video_out(AVFormatContext *s,
     if(ist && ist->st->start_time != AV_NOPTS_VALUE && ist->st->first_dts != AV_NOPTS_VALUE && ost->frame_rate.num)
         duration = 1/(av_q2d(ost->frame_rate) * av_q2d(enc->time_base));
 
+    // We take the conservative approuch here and take the minimum even though
+    // this should be correct on its own but a value too small is harmless, one
+    // too big can lead to errors
+    if (filter->inputs[0]->frame_rate.num > 0 &&
+        filter->inputs[0]->frame_rate.den > 0)
+        duration = FFMIN(duration, 1/(av_q2d(filter->inputs[0]->frame_rate) * av_q2d(enc->time_base)));
+
     if (!ost->filters_script &&
         !ost->filters &&
         next_picture &&