Fix generic wxHeaderCtrl repainting in scrolled state

Clear the device context used for painting the tabs to avoid repainting
issues.

Closes https://github.com/wxWidgets/wxWidgets/pull/908

Closes #18209.

src/generic/headerctrlg.cpp

@@ -490,6 +490,7 @@ void wxHeaderCtrl::OnPaint(wxPaintEvent& WXUNUSED(event))
 #endif
 
     wxAutoBufferedPaintDC dc(this);
+    dc.Clear();
 
     // account for the horizontal scrollbar offset in the parent window
     dc.SetDeviceOrigin(m_scrollOffset, 0);