tdf#91843 sd sidebar: fix unexpected borders around the slide layout previews

Regression from commit e5bb5c52aeaeb60c8592b004efd88c30242e03a1 (add
RenderContext to Window::PushPaintHelper, use Erase correctly,
2015-05-18), the problem was similar to tdf#92079, except here the
missing background was a solid color, not a bitmap.

Change-Id: I2221b3eabed84cf039cb8c3881442c9e23c6ec13
Reviewed-on: https://gerrit.libreoffice.org/78205
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins
diff --git a/vcl/source/window/paint.cxx b/vcl/source/window/paint.cxx
index 8bdbc5a..2d83e307 100644
--- a/vcl/source/window/paint.cxx
+++ b/vcl/source/window/paint.cxx
@@ -298,7 +298,8 @@ void PaintHelper::DoPaint(const vcl::Region* pRegion)
            // direct painting
            Wallpaper aBackground = m_pWindow->GetBackground();
            m_pWindow->ApplySettings(*m_pWindow);
            if (aBackground.IsBitmap())
            // Restore lost bitmap or solid color background.
            if (aBackground.IsBitmap() || aBackground.GetColor() != COL_TRANSPARENT)
                m_pWindow->SetBackground(aBackground);
            m_pWindow->PushPaintHelper(this, *m_pWindow);
            m_pWindow->Paint(*m_pWindow, m_aPaintRect);