Make master preview dirty after rendertexture is resized (fixes #235)

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

             m_PreviewRenderHandle.renderTexture.Release();
             Object.DestroyImmediate(m_PreviewRenderHandle.renderTexture);
             m_PreviewRenderHandle.renderTexture = new RenderTexture(descriptor);
+
+            DirtyMasterNode(ModificationScope.Node);
         }
 
         public void UpdateRenderTextureOnNextLayoutChange()