Avoid computing the difference between two potentially distant iterators, which is O(1) in wchar_t build, but O(n) in UTF-8 one. See #13445. git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71370 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775