lavfi: add const to the AVFilter parameter of avfilter_graph_create_filter()

This function should never modify the filter.

libavfilter/avfilter.h

@@ -1001,7 +1001,7 @@ int avfilter_graph_add_filter(AVFilterGraph *graphctx, AVFilterContext *filter);
  * @return a negative AVERROR error code in case of failure, a non
  * negative value otherwise
  */
-int avfilter_graph_create_filter(AVFilterContext **filt_ctx, AVFilter *filt,
+int avfilter_graph_create_filter(AVFilterContext **filt_ctx, const AVFilter *filt,
                                  const char *name, const char *args, void *opaque,
                                  AVFilterGraph *graph_ctx);
 

libavfilter/avfiltergraph.c

@@ -139,7 +139,7 @@ FF_ENABLE_DEPRECATION_WARNINGS
 }
 #endif
 
-int avfilter_graph_create_filter(AVFilterContext **filt_ctx, AVFilter *filt,
+int avfilter_graph_create_filter(AVFilterContext **filt_ctx, const AVFilter *filt,
                                  const char *name, const char *args, void *opaque,
                                  AVFilterGraph *graph_ctx)
 {