fix rendercontext rendering of standard toolbar icons

(cherry-picked from 177f8e8b)

Change-Id: I6e0399c16d3e900fbca56fc757f342ea893d0ad4
Reviewed-on: https://gerrit.libreoffice.org/16087Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/source/window/paint.cxx

@@ -178,7 +178,7 @@ void PaintHelper::DoPaint(const vcl::Region* pRegion)
 {
     WindowImpl* pWindowImpl = m_pWindow->ImplGetWindowImpl();
     vcl::Region* pWinChildClipRegion = m_pWindow->ImplGetWinChildClipRegion();
-    if (pWindowImpl->mnPaintFlags & IMPL_PAINT_PAINTALL)
+    if (pWindowImpl->mnPaintFlags & IMPL_PAINT_PAINTALL || m_pBuffer)
     {
         pWindowImpl->maInvalidateRegion = *pWinChildClipRegion;
     }