lavfi: deprecate avfilter_graph_add_filter().

Since this function adds a standalone filter to a filter graph and we do
not support creating such filters, there is no reason for this function
to exist.

doc/APIchanges

@@ -16,7 +16,8 @@ API changes, most recent first:
 2013-xx-xx - lavfi 3.8.0
   Move all content from avfiltergraph.h to avfilter.h. Deprecate
   avfilterhraph.h, user applications should include just avfilter.h
-  Add avfilter_graph_alloc_filter(), deprecate avfilter_open().
+  Add avfilter_graph_alloc_filter(), deprecate avfilter_open() and
+  avfilter_graph_add_filter().
 
 2013-xx-xx - lavfi 3.7.0 - avfilter.h
   Add AVFilter.priv_class for exporting filter options through the AVOptions API

libavfilter/avfilter.h

@@ -750,13 +750,19 @@ AVFilterContext *avfilter_graph_alloc_filter(AVFilterGraph *graph,
  */
 AVFilterContext *avfilter_graph_get_filter(AVFilterGraph *graph, char *name);
 
+#if FF_API_AVFILTER_OPEN
 /**
  * Add an existing filter instance to a filter graph.
  *
  * @param graphctx  the filter graph
  * @param filter the filter to be added
+ *
+ * @deprecated use avfilter_graph_alloc_filter() to allocate a filter in a
+ * filter graph
  */
+attribute_deprecated
 int avfilter_graph_add_filter(AVFilterGraph *graphctx, AVFilterContext *filter);
+#endif
 
 /**
  * Create and add a filter instance into an existing graph.

libavfilter/avfiltergraph.c

@@ -58,6 +58,7 @@ void avfilter_graph_free(AVFilterGraph **graph)
     av_freep(graph);
 }
 
+#if FF_API_AVFILTER_OPEN
 int avfilter_graph_add_filter(AVFilterGraph *graph, AVFilterContext *filter)
 {
     AVFilterContext **filters = av_realloc(graph->filters,
@@ -74,6 +75,7 @@ int avfilter_graph_add_filter(AVFilterGraph *graph, AVFilterContext *filter)
 
     return 0;
 }
+#endif
 
 int avfilter_graph_create_filter(AVFilterContext **filt_ctx, AVFilter *filt,
                                  const char *name, const char *args, void *opaque,