Avoid using double buffering unnecessarily in wxAuiNotebook

We don't need to do double buffering on our own if the underlying
platform always does it anyhow, as macOS and GTK 3 do.

src/aui/auibook.cpp

@@ -21,6 +21,7 @@
 
 #ifndef WX_PRECOMP
     #include "wx/settings.h"
+    #include "wx/dcbuffer.h" // just for wxALWAYS_NATIVE_DOUBLE_BUFFER
     #include "wx/dcclient.h"
     #include "wx/dcmemory.h"
     #include "wx/frame.h"
@@ -414,6 +415,13 @@ void wxAuiTabContainer::Render(wxDC* raw_dc, wxWindow* wnd)
     if (m_rect.IsEmpty())
         return;
 
+    size_t i;
+    size_t page_count = m_pages.GetCount();
+    size_t button_count = m_buttons.GetCount();
+
+#if wxALWAYS_NATIVE_DOUBLE_BUFFER
+    wxDC& dc = *raw_dc;
+#else
     wxMemoryDC dc;
 
     // use the same layout direction as the window DC uses to ensure that the
@@ -421,16 +429,13 @@ void wxAuiTabContainer::Render(wxDC* raw_dc, wxWindow* wnd)
     dc.SetLayoutDirection(raw_dc->GetLayoutDirection());
 
     wxBitmap bmp;
-    size_t i;
-    size_t page_count = m_pages.GetCount();
-    size_t button_count = m_buttons.GetCount();
-
     // create off-screen bitmap
     bmp.Create(m_rect.GetWidth(), m_rect.GetHeight(),*raw_dc);
     dc.SelectObject(bmp);
 
     if (!dc.IsOk())
         return;
+#endif
 
     // ensure we show as many tabs as possible
     while (m_tabOffset > 0 && IsTabVisible(page_count-1, m_tabOffset-1, &dc, wnd))
@@ -707,9 +712,11 @@ void wxAuiTabContainer::Render(wxDC* raw_dc, wxWindow* wnd)
     }
 
 
+#if !wxALWAYS_NATIVE_DOUBLE_BUFFER
     raw_dc->Blit(m_rect.x, m_rect.y,
                  m_rect.GetWidth(), m_rect.GetHeight(),
                  &dc, 0, 0);
+#endif
 }
 
 // Is the tab visible?