Implement avfilter_graph_alloc().

Originally committed as revision 25673 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavfilter/avfiltergraph.h

@@ -31,6 +31,11 @@ typedef struct AVFilterGraph {
     char *scale_sws_opts; ///< sws options to use for the auto-inserted scale filters
 } AVFilterGraph;
 
+/**
+ * Allocate a filter graph.
+ */
+AVFilterGraph *avfilter_graph_alloc(void);
+
 /**
  * Get a filter instance with name name from graph.
  *