Fix glitch in AUI frame with standard wxToolbar

When the height of the AUI toolbar pane is higher than the wxToolbar, the extra
area shows a glitch. This happens because the paint handler never draws on this
area. Clearing the DC of the frame with the AUI background colour fixes this.

See https://github.com/wxWidgets/wxWidgets/pull/829

Closes #18138

src/aui/framemanager.cpp

@@ -3935,6 +3935,10 @@ void wxAuiManager::Repaint(wxDC* dc)
 void wxAuiManager::OnPaint(wxPaintEvent& WXUNUSED(event))
 {
     wxPaintDC dc(m_frame);
+
+    dc.SetBackground(GetArtProvider()->GetColor(wxAUI_DOCKART_BACKGROUND_COLOUR));
+    dc.Clear();
+
     Repaint(&dc);
 }