ffmpeg: Force a first_pts of 0 for the first configuration of -async use

This is very close to what the default does currently
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.h

@@ -198,6 +198,7 @@ typedef struct FilterGraph {
     const char    *graph_desc;
 
     AVFilterGraph *graph;
+    int reconfiguration;
 
     InputFilter   **inputs;
     int          nb_inputs;

ffmpeg_filter.c

@@ -675,6 +675,8 @@ static int configure_input_audio_filter(FilterGraph *fg, InputFilter *ifilter,
         av_strlcatf(args, sizeof(args), "async=%d", audio_sync_method);
         if (audio_drift_threshold != 0.1)
             av_strlcatf(args, sizeof(args), ":min_hard_comp=%f", audio_drift_threshold);
+        if (!fg->reconfiguration)
+            av_strlcatf(args, sizeof(args), ":first_pts=0");
         AUTO_INSERT_FILTER_INPUT("-async", "aresample", args);
     }
 
@@ -796,6 +798,7 @@ int configure_filtergraph(FilterGraph *fg)
         }
     }
 
+    fg->reconfiguration = 1;
     return 0;
 }