avfilter/avfilter: Add avfilter_filter_pad_count()

It is intended as replacement for avfilter_pad_count(). In contrast to
the latter, it avoids a loop.

Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavfilter/avfilter.h

@@ -325,6 +325,11 @@ typedef struct AVFilter {
     int (*activate)(AVFilterContext *ctx);
 } AVFilter;
 
+/**
+ * Get the number of elements in an AVFilter's inputs or outputs array.
+ */
+unsigned avfilter_filter_pad_count(const AVFilter *filter, int is_output);
+
 /**
  * Process multiple parts of the frame concurrently.
  */