No changes, just refactor wxMSW background brush methods.

Factor out MSWGetCustomBgBrush() from MSWGetBgBrushForChild(). This is useful
as in the vast majority of cases the parent window will want to use the same
background brush for all of its children so it doesn't really care about the
concrete child passed to MSWGetBgBrushForChild() and we can adjust the brush
to the child origin in the common code instead of asking each derived class
overriding MSWGetBgBrushForChild() to do this.

This doesn't change anything but will make the upcoming changes to wxPanel
background painting simpler.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67250 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/notebook.h

@@ -142,9 +142,6 @@ public:
       return true;
   }
 
-  // return the themed brush for painting our children
-  virtual WXHBRUSH MSWGetBgBrushForChild(WXHDC hDC, wxWindow *child);
-
   // draw child background
   virtual bool MSWPrintChild(WXHDC hDC, wxWindow *win);
 
@@ -174,6 +171,9 @@ protected:
   void AdjustPageSize(wxNotebookPage *page);
 
 #if wxUSE_UXTHEME
+  // return the themed brush for painting our children
+  virtual WXHBRUSH MSWGetCustomBgBrush() { return m_hbrBackground; }
+
   // gets the bitmap of notebook background and returns a brush from it
   WXHBRUSH QueryBgBitmap();