ffmpeg: Favor filter graph based duration over ost->frame_rate in fps code

The fate change adds a last frame with a subtitle not displayed in the previous
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -900,15 +900,12 @@ static void do_video_out(AVFormatContext *s,
     if (ost->source_index >= 0)
         ist = input_streams[ost->source_index];
 
-    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)));
+        duration = 1/(av_q2d(filter->inputs[0]->frame_rate) * av_q2d(enc->time_base));
+
+    if(ist && ist->st->start_time != AV_NOPTS_VALUE && ist->st->first_dts != AV_NOPTS_VALUE && ost->frame_rate.num)
+        duration = FFMIN(duration, 1/(av_q2d(ost->frame_rate) * av_q2d(enc->time_base)));
 
     if (!ost->filters_script &&
         !ost->filters &&

tests/ref/fate/sub2video

@@ -52,6 +52,7 @@
 0,         47,         47,        1,   518400, 0xde69683f
 0,         48,         48,        1,   518400, 0x7df08fba
 0,         49,         49,        1,   518400, 0xbab197ea
+0,         50,         50,        1,   518400, 0x902285d9
 1,      15355,      15355,     4733,     2094, 0x3c171425, F=0x0
 1,      48797,      48797,     2560,     2480, 0x7c0edf21, F=0x0
 1,      51433,      51433,     2366,     3059, 0xc95b8a05, F=0x0