fix for case #987330 now the preview is behind the outline.

com.unity.shadergraph/Editor/Drawing/Views/MaterialNodeView.cs

                 if (m_PreviewContainer.parent != this)
                 {
                     Add(m_PreviewContainer);
+                    m_PreviewContainer.PlaceBehind(this.Q("selection-border"));
                 }
                 m_PreviewFiller.AddToClassList("expanded");
                 m_PreviewFiller.RemoveFromClassList("collapsed");