Resolves: tdf#152704 old image remains underneath when new image drawn

in this dialog preview widget

Change-Id: Id33995d9a9557d235178be0d12cc6b44e146ff4c
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/148997
Tested-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
diff --git a/sw/source/ui/frmdlg/frmpage.cxx b/sw/source/ui/frmdlg/frmpage.cxx
index e9cf997..94c86e9 100644
--- a/sw/source/ui/frmdlg/frmpage.cxx
+++ b/sw/source/ui/frmdlg/frmpage.cxx
@@ -2658,7 +2658,8 @@ void BmpWindow::SetDrawingArea(weld::DrawingArea* pDrawingArea)
void BmpWindow::Paint(vcl::RenderContext& rRenderContext, const tools::Rectangle&)
{
    // Setup
    rRenderContext.SetBackground();
    rRenderContext.SetBackground(Wallpaper(Application::GetSettings().GetStyleSettings().GetDialogColor()));
    rRenderContext.Erase();
    // #i119307# the graphic might have transparency, set up white as the color
    // to use when drawing a rectangle under the image
    rRenderContext.SetLineColor(COL_WHITE);