tdf#125413 vcl menu bar window: fix non-NWF background

Windows (by default) and gtk3 paints its own background, but e.g. the
Linux gen backend does not; so make sure that we not only copy from the
buffer, but also initialize it.

This restores the gradient background of the main menu with the Linux
gen backend.

Change-Id: I5ce8cc734f64bc1d57d343caf22071e6aa63a69f
Reviewed-on: https://gerrit.libreoffice.org/72676
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>

vcl/source/window/menubarwindow.cxx

@@ -916,6 +916,9 @@ void MenuBarWindow::Paint(vcl::RenderContext& rRenderContext, const tools::Recta
     // Make sure that all actual rendering happens in one go to avoid flicker.
     ScopedVclPtrInstance<VirtualDevice> pBuffer;
     pBuffer->SetOutputSizePixel(aOutputSize, false);
+    // Copy the current state to the buffer.
+    pBuffer->DrawOutDev(Point(0, 0), GetOutputSizePixel(), Point(0, 0), GetOutputSizePixel(),
+                        rRenderContext);
 
     if (rRenderContext.IsNativeControlSupported(ControlType::Menubar, ControlPart::Entire))
     {
@@ -959,6 +962,7 @@ void MenuBarWindow::Paint(vcl::RenderContext& rRenderContext, const tools::Recta
         pBuffer->Pop();
     }
 
+    // Copy the current state from the buffer.
     rRenderContext.DrawOutDev(Point(0, 0), GetOutputSizePixel(), Point(0, 0), GetOutputSizePixel(),
                               *pBuffer);
 }