Use stock wxBrush

Instead of using dynamically created brushes (implicitly from stock colours)
we should use corresponding stock brushes because once created they are
stored in the cache what can slightly improve performance.

src/osx/nonownedwnd_osx.cpp

@@ -544,11 +544,11 @@ bool wxNonOwnedWindow::DoSetPathShape(const wxGraphicsPath& path)
 
     {
         wxMemoryDC dc(bmp);
-        dc.SetBackground(*wxBLACK);
+        dc.SetBackground(*wxBLACK_BRUSH);
         dc.Clear();
 
         wxScopedPtr<wxGraphicsContext> context(wxGraphicsContext::Create(dc));
-        context->SetBrush(*wxWHITE);
+        context->SetBrush(*wxWHITE_BRUSH);
         context->SetAntialiasMode(wxANTIALIAS_NONE);
         context->FillPath(path);
     }