Add mesh selection for master node preveiw

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/Inspector/GraphInspectorView.cs

         VisualElement m_PropertyItems;
         VisualElement m_LayerItems;
         VisualElement m_ContentContainer;
+        Experimental.UIElements.ObjectField m_PreviewMeshPicker;
         AbstractNodeEditorView m_EditorView;
 
         TypeMapper m_TypeMapper;
 
                 m_Preview = new PreviewView {name = "preview", image = Texture2D.blackTexture};
                 bottomContainer.Add(m_Preview);
+
+                m_PreviewMeshPicker = new Experimental.UIElements.ObjectField() { objectType = typeof(Mesh) };
+                bottomContainer.Add(m_PreviewMeshPicker);
             }
             Add(bottomContainer);
 
                 if (m_MasterNode != null)
                 {
                     m_PreviewHandle = m_PreviewSystem.GetPreview(m_MasterNode);
+                    m_PreviewHandle.mesh = null;
                     m_PreviewHandle.onPreviewChanged += OnPreviewChanged;
                 }
             }
         void OnPreviewChanged()
         {
             m_Preview.image = m_PreviewHandle.texture ?? Texture2D.blackTexture;
+            m_PreviewHandle.mesh = m_PreviewMeshPicker.value as Mesh;
         }
 
         public void UpdateSelection(IEnumerable<INode> nodes)

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/MaterialGraphPreviewGenerator.cs

             }
         }
 
-        public void DoRenderPreview(RenderTexture renderTexture, Material mat, PreviewMode mode, bool allowSRP, float time, MaterialPropertyBlock properties = null)
+        public void DoRenderPreview(RenderTexture renderTexture, Material mat, Mesh mesh, PreviewMode mode, bool allowSRP, float time, MaterialPropertyBlock properties = null)
         {
             if (mat == null || mat.shader == null)
                 return;
             Light1.intensity = 1.0f;
             m_Camera.clearFlags = CameraClearFlags.Depth;
 
+            Mesh previewMesh = mesh == null ? s_Meshes[0] : mesh;
+
-                mode == PreviewMode.Preview3D ? s_Meshes[0] : quad,
+                mode == PreviewMode.Preview3D ? previewMesh : quad,
                 Matrix4x4.TRS(Vector3.zero, Quaternion.identity, Vector3.one),
                 mat,
                 1,

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/NodeInspectors/BasicNodeInspector.cs

             var modified = false;
             foreach (var slot in node.GetSlots<MaterialSlot>())
                 modified |= IMGUISlotEditorView.SlotField(slot);
-                
+
             GUILayout.Space(10);
 
             return modified ? ModificationScope.Node : ModificationScope.Nothing;

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/PreviewSystem.cs

                 }
                 var node = m_Graph.GetNodeFromGuid(nodeGuid);
                 m_PreviewMaterial.shader = previewData.shader;
-                m_PreviewGenerator.DoRenderPreview(previewData.renderTexture, m_PreviewMaterial, previewData.previewMode, node is IMasterNode, time, m_PreviewPropertyBlock);
+                m_PreviewGenerator.DoRenderPreview(previewData.renderTexture, m_PreviewMaterial, previewData.mesh, previewData.previewMode, node is IMasterNode, time, m_PreviewPropertyBlock);
                 previewData.texture = previewData.renderTexture;
             }
 
     public class PreviewData
     {
         public Shader shader { get; set; }
+        public Mesh mesh { get; set; }
         public string shaderString { get; set; }
         public PreviewMode previewMode { get; set; }
         public RenderTexture renderTexture { get; set; }

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Testing/IntegrationTests/ShaderGenerationTest.cs

             using (var generator = new MaterialGraphPreviewGenerator())
             {
                 var renderTexture = new RenderTexture(res, res, 16, RenderTextureFormat.ARGB32, RenderTextureReadWrite.Default) { hideFlags = HideFlags.HideAndDontSave };
-                generator.DoRenderPreview(renderTexture, m_PreviewMaterial, PreviewMode.Preview3D, true, 10);
+                generator.DoRenderPreview(renderTexture, m_PreviewMaterial, null, PreviewMode.Preview3D, true, 10);
 
                 Assert.IsNotNull(renderTexture, "Render failed");