Update shape fill preview on switch to this tab

(related to i#95158, but not quite)
diff --git a/cui/source/tabpages/tpshadow.cxx b/cui/source/tabpages/tpshadow.cxx
index a7109fa..086f4f4 100644
--- a/cui/source/tabpages/tpshadow.cxx
+++ b/cui/source/tabpages/tpshadow.cxx
@@ -245,6 +245,7 @@ void SvxShadowTabPage::ActivatePage( const SfxItemSet& rSet )
                    }
                }

                aCtlXRectPreview.SetRectangleAttributes(rSet);
                // aLbShadowColor
                nPos = aLbShadowColor.GetSelectEntryPos();
                aLbShadowColor.Clear();