Fix flicker when resizing columns of report-mode MSW wxListCtrl

Just turn off background erasing to avoid having horrible flicker which
can be seen perfectly well simply by drag-resizing a column in a list
control with non-default background colour.

See https://github.com/wxWidgets/wxWidgets/pull/374

src/msw/listctrl.cpp

@@ -334,6 +334,10 @@ void wxListCtrl::MSWSetExListStyles()
         // it seems better to enable it by default than disable
         LVS_EX_HEADERDRAGDROP
     );
+
+    // As we use LVS_EX_DOUBLEBUFFER above, we don't need to erase our
+    // background and doing it only results in flicker.
+    SetBackgroundStyle(wxBG_STYLE_PAINT);
 }
 
 WXDWORD wxListCtrl::MSWGetStyle(long style, WXDWORD *exstyle) const