avfilter/dualinput: Add support to skip initial unpaired frames

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/dualinput.c

@@ -66,6 +66,9 @@ int ff_dualinput_init(AVFilterContext *ctx, FFDualInputContext *s)
         in[1].after = EXT_NULL;
         in[1].sync  = 0;
     }
+    if (s->skip_initial_unpaired) {
+        in[1].before = EXT_STOP;
+    }
 
     return ff_framesync_configure(&s->fs);
 }

libavfilter/dualinput.h

@@ -35,6 +35,7 @@ typedef struct {
     AVFrame *(*process)(AVFilterContext *ctx, AVFrame *main, const AVFrame *second);
     int shortest;               ///< terminate stream when the second input terminates
     int repeatlast;             ///< repeat last second frame
+    int skip_initial_unpaired;  ///< Skip initial frames that do not have a 2nd input
 } FFDualInputContext;
 
 int ff_dualinput_init(AVFilterContext *ctx, FFDualInputContext *s);