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

src/msw/notebook.cpp

@@ -819,14 +819,6 @@ bool wxNotebook::InsertPage(size_t nPage,
     // succeeded: save the pointer to the page
     m_pages.Insert(pPage, nPage);
 
-    // also ensure that the notebook background is used for its pages by making
-    // them transparent: this ensures that MSWGetBgBrush() queries the notebook
-    // for the background brush to be used for erasing them
-    if ( wxPanel *panel = wxDynamicCast(pPage, wxPanel) )
-    {
-        panel->MSWSetTransparentBackground();
-    }
-
     // we may need to adjust the size again if the notebook size changed:
     // normally this only happens for the first page we add (the tabs which
     // hadn't been there before are now shown) but for a multiline notebook it