Make SubGraphOutputNode have 3D preview similar to MasterNodes

com.unity.shadergraph/Editor/Data/SubGraph/SubGraphOutputNode.cs

             get { return true; }
         }
 
+        public override PreviewMode previewMode
+        {
+            get { return PreviewMode.Preview3D; }
+        }
+
         public virtual int AddSlot()
         {
             var index = this.GetInputSlots<ISlot>().Count() + 1;