[Material graph] Fix control id issues when there are two nodes of the same type

MaterialGraphProject/Assets/GraphFramework/SerializableGraph/Editor/Drawing/MaterialNodeData.cs

     {
         public override void OnGUIHandler()
         {
+			base.OnGUIHandler ();
+
+
             cNode.color =  EditorGUILayout.ColorField("Color", cNode.color);
         }
     }

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/NodeDrawers/AbstractMaterialNodeUI.cs

             node = inNode;
         }
 
-        public abstract void OnGUIHandler();
+        public virtual void OnGUIHandler()
+		{
+			if (node == null)
+				return;
+			
+			GUIUtility.GetControlID (node.guid.GetHashCode(), FocusType.Passive);
+		}
     }
 
     [Serializable]