Inherit notebook background recursively under wxMSW.

With MSWSetTransparentBackground() hack only the panel which was the immediate
child of wxNotebook (i.e. its page) inherited the notebook background but not
its children. This resulted in jarring background discontinuities when nested
panels were used.

Fix this by inheriting notebook background in all child panels by testing for
the return value of the parents MSWHasInheritableBackground() method in
wxPanel::HasTransparentBackground() recursively.

Closes #12317.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65238 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/window.h

@@ -438,6 +438,11 @@ public:
         return true;
     }
 
+    // This should be overridden to return true for the controls which have
+    // themed background that should through their children. Currently only
+    // wxNotebook uses this.
+    virtual bool MSWHasInheritableBackground() const { return false; }
+
 #if !defined(__WXWINCE__) && !defined(__WXUNIVERSAL__)
     #define wxHAS_MSW_BACKGROUND_ERASE_HOOK
 #endif