initialize backing store bitmap in Create(), notebook doesn't always get a size event immediately after creation

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@35121 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/notebook.cpp

@@ -345,6 +345,11 @@ bool wxNotebook::Create(wxWindow *parent,
     {
         SetBackgroundColour(GetThemeBackgroundColour());
     }
+    else // use themed background by default
+    {
+        // create backing store
+        UpdateBgBrush();
+    }
 #endif // wxUSE_UXTHEME
 
     // Undocumented hack to get flat notebook style