Fixed refresh bug in border preview control when resizing

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75238 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/richtext/richtextborderspage.cpp

@@ -1235,6 +1235,9 @@ void wxRichTextBorderPreviewCtrl::OnPaint(wxPaintEvent& WXUNUSED(event))
 {
     wxPaintDC dc(this);
 
+    dc.SetBackground(*wxWHITE_BRUSH);
+    dc.Clear();
+
     if (m_attributes)
     {
         wxRect rect = GetClientRect();