VclContainer: use transparent background

see http://lists.freedesktop.org/archives/libreoffice/2012-November/040587.html,
screenshot at [3].

Change-Id: Iaf72b3734564301244365773f05c6df01a6dea62
Reviewed-on: https://gerrit.libreoffice.org/1143Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/source/window/layout.cxx

@@ -16,6 +16,7 @@ VclContainer::VclContainer(Window *pParent)
     , m_bLayoutDirty(true)
 {
     ImplInit(pParent, WB_HIDE, NULL);
+    SetBackground();
 }
 
 Size VclContainer::GetOptimalSize(WindowSizeType eType) const