rendercontext: Fix rendering of double-buffered menubar.

Change-Id: Iea74564ef0ae2188ae29f106c4416df1fe827b42

vcl/source/window/menu.cxx

@@ -1822,7 +1822,9 @@ void Menu::ImplPaint(vcl::RenderContext& rRenderContext,
         aTopLeft.Y() += ImplGetSVData()->maNWFData.mnMenuFormatBorderY;
     }
 
-    Size aOutSz = rRenderContext.GetOutputSizePixel();
+    // for the computations, use size of the underlying window, not of RenderContext
+    Size aOutSz = pWindow->GetOutputSizePixel();
+
     size_t nCount = pItemList->size();
     if (bLayout)
         mpLayoutData->m_aVisibleItemBoundRects.clear();