Simplify and correct MSW selection of background brush for children painting.

Check for the transparency of the window background in MSWGetBgBrush() itself
and let MSWGetBgBrushForChild() just return the brush to be used.

This required adding a MSW-specific hack to wxPanel to allow inheriting
background of wxNotebook by its children which is slightly ugly but is exactly
what we need to do in this case and makes the rest of the code much simpler.

Also add a possibility to set the background of the page containing the
control being tested to the widgets sample to test for background inheritance.
This shows problems with wxSlider and wxRadioBox backgrounds but they're not
new with this patch.

Closes #12057.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64372 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/generic/panelg.h

@@ -76,6 +76,20 @@ public:
     virtual bool IsCanvasWindow() const { return true; }
 #endif
 
+#ifdef __WXMSW__
+    // This is a hack to support inheriting of background through child
+    // wxPanel: at least wxNotebook needs this under wxMSW as its background
+    // should apply to its children which are usually wxPanels which normally
+    // don't have a transparent background. Calling this function allows to
+    // change this for the panels which are used as notebook pages.
+    void MSWSetTransparentBackground(bool isTransparent = true)
+    {
+        m_isTransparent = isTransparent;
+    }
+
+    virtual bool HasTransparentBackground() { return m_isTransparent; }
+#endif // __WXMSW__
+
     WX_DECLARE_CONTROL_CONTAINER();
 
 protected:
@@ -85,6 +99,11 @@ protected:
     // choose the default border for this window
     virtual wxBorder GetDefaultBorder() const { return wxWindowBase::GetDefaultBorder(); }
 
+private:
+#ifdef __WXMSW__
+    bool m_isTransparent;
+#endif // __WXMSW__
+
     DECLARE_DYNAMIC_CLASS_NO_COPY(wxPanel)
     DECLARE_EVENT_TABLE()
 };