Resolves: tdf#121946 first time draw uses gray bg

Change-Id: Ia0491eebe8fa7ea2c2377ca96c758c681da5470a
Reviewed-on: https://gerrit.libreoffice.org/64875
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

svx/source/dialog/charmap.cxx

@@ -623,6 +623,7 @@ void SvxShowCharSet::InitSettings(vcl::RenderContext& rRenderContext)
     if (mbUpdateBackground)
     {
         rRenderContext.SetBackground(rStyleSettings.GetWindowColor());
+        rRenderContext.Erase();
         mbUpdateBackground = false;
     }