tdf#170406 Thumbnails for some documents rendered with artifacts

regression from
  commit 07fd24a4bd
  Author: Noel Grandin <noel.grandin@collabora.co.uk>
  Date:   Sat Aug 2 09:15:36 2025 +0200
  BitmapEx->Bitmap in ThumbnailView

Change-Id: I24ffd28e789f904c999b2759df41f2e15f328e0f
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/200790
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
Tested-by: Jenkins

sfx2/source/control/recentdocsviewitem.cxx

@@ -203,6 +203,7 @@ RecentDocsViewItem::RecentDocsViewItem(sfx2::RecentDocsView &rView, const OUStri
 
         // create empty, and copy the default thumbnail in
         aThumbnail = Bitmap(aThumbnailSize, vcl::PixelFormat::N32_BPP);
+        aThumbnail.Erase(COL_TRANSPARENT);
 
         aThumbnail.CopyPixel(
                 ::tools::Rectangle(Point((aThumbnailSize.Width() - aExtSize.Width()) / 2, (aThumbnailSize.Height() - aExtSize.Height()) / 2), aExtSize),