utf8.c: Don't throw away a value and then recalc it

utf8.c: Don't throw away a value and then recalc it

In half the calls to to_utf8_case(), the code point being looked up is
known.  It is thrown away because the API doesn't pass it, and then
recalculated first thing in to_utf8_case.

Fix this by making a new static function which adds the code point to
the parameter list, and change all calls to use this, leaving the
existing to_utf8_case() as just a wrapper for the new function.