WINTERMUTE: Allow utf8ToWide() and wideToUtf8() work with ASCII strings

This is needed for English versions of multilingual games, which use
UTF-8 strings, but we can treat them as plain ASCII, since wide and
UTF-8 strings are not yet supported in Wintermute. This allows at least
the English versions of these games to run.

This allows Reversion 2 and Shaban to start