Fix T44213: Bevel object from different scene won't update generated bezier curve geometry

The issue was caused by bevel object being automatically added to the scene graph
by dag_get_node() and had no incoming relations, even form the scene. This confused
scene update flush logic.

Now there'll be a scene relation added to such nodes, so they're always reachable
from the root node.

source/blender/blenkernel/intern/depsgraph.c

@@ -960,6 +960,10 @@ DagForest *build_dag(Main *bmain, Scene *sce, short mask)
 
 			/* also flush custom data mask */
 			((Object *)node->ob)->customdata_mask = node->customdata_mask;
+
+			if (node->parent == NULL) {
+				dag_add_relation(dag, scenenode, node, DAG_RL_SCENE, "Scene Relation");
+			}
 		}
 	}
 	/* now set relations equal, so that when only one parent changes, the correct recalcs are found */