lavfi: add a function for counting elements in AVFilterPad arrays.

The caller needs to know what valid indices can be passed to
avfilter_pad_get_name/type.

doc/APIchanges

@@ -23,6 +23,7 @@ API changes, most recent first:
   Add avfilter_init_str(), deprecate avfilter_init_filter().
   Add avfilter_init_dict().
   Add AVFilter.flags field and AVFILTER_FLAG_DYNAMIC_{INPUTS,OUTPUTS} flags.
+  Add avfilter_pad_count() for counting filter inputs/outputs.
 
 2013-xx-xx - lavfi 3.7.0 - avfilter.h
   Add AVFilter.priv_class for exporting filter options through the AVOptions API

libavfilter/avfilter.c

@@ -304,7 +304,7 @@ void avfilter_uninit(void)
     next_registered_avfilter_idx = 0;
 }
 
-static int pad_count(const AVFilterPad *pads)
+int avfilter_pad_count(const AVFilterPad *pads)
 {
     int count;
 
@@ -377,7 +377,7 @@ AVFilterContext *ff_filter_alloc(const AVFilter *filter, const char *inst_name)
         av_opt_set_defaults(ret->priv);
     }
 
-    ret->nb_inputs = pad_count(filter->inputs);
+    ret->nb_inputs = avfilter_pad_count(filter->inputs);
     if (ret->nb_inputs ) {
         ret->input_pads   = av_malloc(sizeof(AVFilterPad) * ret->nb_inputs);
         if (!ret->input_pads)
@@ -388,7 +388,7 @@ AVFilterContext *ff_filter_alloc(const AVFilter *filter, const char *inst_name)
             goto err;
     }
 
-    ret->nb_outputs = pad_count(filter->outputs);
+    ret->nb_outputs = avfilter_pad_count(filter->outputs);
     if (ret->nb_outputs) {
         ret->output_pads  = av_malloc(sizeof(AVFilterPad) * ret->nb_outputs);
         if (!ret->output_pads)

libavfilter/avfilter.h

@@ -350,6 +350,12 @@ struct AVFilterPad {
 };
 #endif
 
+/**
+ * Get the number of elements in a NULL-terminated array of AVFilterPads (e.g.
+ * AVFilter.inputs/outputs).
+ */
+int avfilter_pad_count(const AVFilterPad *pads);
+
 /**
  * Get the name of an AVFilterPad.
  *