because some sash code draws whether the window is visible or not, the easiest thing...

because some sash code draws whether the window is visible or not, the easiest thing is to make the MacGetVisibleRegion return an empty region for this case, so the wxDC is always entirely clipped

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@25710 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775