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/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;