Depsgraph: Add utility function to get depsgraph from node handle

The idea is to use this function for modifiers' updateDepsgraph functions
instead of doing direct scene->depsgraph access.

source/blender/depsgraph/DEG_depsgraph_build.h

@@ -153,6 +153,8 @@ void DEG_add_object_cache_relation(struct DepsNodeHandle *handle,
                                    eDepsObjectComponentType component,
                                    const char *description);
 
+
+struct Depsgraph *DEG_get_graph_from_handle(struct DepsNodeHandle *handle);
 void DEG_add_special_eval_flag(struct Depsgraph *graph, struct ID *id, short flag);
 
 /* Utility functions for physics modifiers */

source/blender/depsgraph/intern/builder/deg_builder_relations.cc

@@ -371,6 +371,11 @@ void DepsgraphRelationBuilder::add_forcefield_relations(const OperationKey &key,
 	pdEndEffectors(&effectors);
 }
 
+Depsgraph *DepsgraphRelationBuilder::getGraph()
+{
+	return m_graph;
+}
+
 /* **** Functions to build relations between entities  **** */
 
 void DepsgraphRelationBuilder::begin_build(Main *bmain)

source/blender/depsgraph/intern/builder/deg_builder_relations.h

@@ -233,6 +233,8 @@ struct DepsgraphRelationBuilder
 	template <typename KeyType>
 	OperationDepsNode *find_operation_node(const KeyType &key);
 
+	Depsgraph *getGraph();
+
 protected:
 	RootDepsNode *find_node(const RootKey &key) const;
 	TimeSourceDepsNode *find_node(const TimeSourceKey &key) const;

source/blender/depsgraph/intern/depsgraph_build.cc

@@ -166,6 +166,13 @@ void DEG_add_bone_relation(DepsNodeHandle *handle,
 	                                              description);
 }
 
+struct Depsgraph *DEG_get_graph_from_handle(struct DepsNodeHandle *handle)
+{
+	DEG::DepsNodeHandle *deg_handle = get_handle(handle);
+	DEG::DepsgraphRelationBuilder *relation_builder = deg_handle->builder;
+	return reinterpret_cast<Depsgraph *>(relation_builder->getGraph());
+}
+
 void DEG_add_special_eval_flag(Depsgraph *graph, ID *id, short flag)
 {
 	DEG::Depsgraph *deg_graph = reinterpret_cast<DEG::Depsgraph *>(graph);