Depsgraph: Bring timing statistics to the new dependency graph

This statistics is only collected when debug_value is different from 0.

Stored in depsgraph node itself, so we can always have access to average data
and other stats which requires persistent storage. This way we also don't waste
time trying to find stats from a separately stored hash map.

source/blender/depsgraph/DEG_depsgraph_debug.h

@@ -55,6 +55,11 @@ void DEG_debug_relations_graphviz(const struct Depsgraph *graph,
                                   FILE *stream,
                                   const char *label);
 
+void DEG_debug_stats_gnuplot(const struct Depsgraph *graph,
+                             FILE *stream,
+                             const char *label,
+                             const char *output_filename);
+
 /* ************************************************ */
 
 /* Compare two dependency graphs. */