[shader graph]Simplify creation of pixel master node.

UnityProject/Assets/UnityShaderEditor/Editor/Source/BaseMaterialGraph.cs

 
     public abstract class BaseMaterialGraph : Graph
     {
-        public abstract BaseMaterialNode masterNode { get; }
-
         private PreviewRenderUtility m_PreviewUtility;
 
         public PreviewRenderUtility previewUtility

UnityProject/Assets/UnityShaderEditor/Editor/Source/MaterialGraph.cs

                 m_Shader.hideFlags = HideFlags.HideInHierarchy;
             }
             AssetDatabase.AddObjectToAsset(m_Shader, this);
-            m_PixelGraph.AddMasterNodeToAsset();
-        public UnityEngine.Material GetMaterial()
+        public Material GetMaterial()
         {
             if (m_PixelGraph == null)
                 return null;

UnityProject/Assets/UnityShaderEditor/Editor/Source/MaterialSubGraph.cs

 
         public SubGraphOutputsNode outputsNode { get { return m_OutputsNode; } }
 
-        public override BaseMaterialNode masterNode { get { return outputsNode; } }
         protected override void RecacheActiveNodes()
         {
         }

UnityProject/Assets/UnityShaderEditor/Editor/Source/PixelGraph.cs

 
         public PreviewState previewState { get; set; }
 
-        public override BaseMaterialNode masterNode
-        {
-            get { return pixelMasterNode; }
-        }
-
-        public override void OnEnable()
-        {
-            base.OnEnable();
-
-            if (m_PixelMasterNode == null)
-                m_PixelMasterNode = nodes.FirstOrDefault(x => x.GetType() == typeof (PixelShaderNode)) as PixelShaderNode;
-
-            if (m_PixelMasterNode == null)
-            {
-                m_PixelMasterNode = CreateInstance<PixelShaderNode>();
-                m_PixelMasterNode.hideFlags = HideFlags.HideInHierarchy;
-                m_PixelMasterNode.Init();
-                m_PixelMasterNode.position = new Rect(700, m_PixelMasterNode.position.y, m_PixelMasterNode.position.width, m_PixelMasterNode.position.height);
-                AddMasterNodeNoAddToAsset(m_PixelMasterNode);
-            }
-        }
-
         public PixelShaderNode pixelMasterNode
         {
             get
             }
 
             pixelMasterNode.GenerateNodeCode(shaderBody, genMode);
-        }
-
-        public void AddMasterNodeToAsset()
-        {
-            AssetDatabase.AddObjectToAsset(pixelMasterNode, this);
         }
 
         protected override void RecacheActiveNodes()