lavfi: add AVFilterContext.graph.

It will be useful in the following commits.

doc/APIchanges

@@ -18,6 +18,8 @@ API changes, most recent first:
   avfilterhraph.h, user applications should include just avfilter.h
   Add avfilter_graph_alloc_filter(), deprecate avfilter_open() and
   avfilter_graph_add_filter().
+  Add AVFilterContext.graph pointing to the AVFilterGraph that contains the
+  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

@@ -458,6 +458,8 @@ struct AVFilterContext {
     unsigned    nb_outputs;         ///< number of output pads
 
     void *priv;                     ///< private data for use by the filter
+
+    struct AVFilterGraph *graph;    ///< filtergraph this filter belongs to
 };
 
 /**

libavfilter/avfiltergraph.c

@@ -73,6 +73,8 @@ int avfilter_graph_add_filter(AVFilterGraph *graph, AVFilterContext *filter)
     graph->filter_count = graph->nb_filters;
 #endif
 
+    filter->graph = graph;
+
     return 0;
 }
 #endif
@@ -121,6 +123,8 @@ AVFilterContext *avfilter_graph_alloc_filter(AVFilterGraph *graph,
     graph->filter_count = graph->nb_filters;
 #endif
 
+    s->graph = graph;
+
     return s;
 }