Direct all messages and listings into stderr, since PNG data will be in stdout.
Treat input char as utf8.

As suggested by FreeType 2 tutorial, FT_Get_Char_Index expects a utf32
character if default charmap is unicode charmap.

Thus, I fetch first utf8 character of argv[1] with 'utf8 rule' written
in Wikipedia. Then, the character is converted into utf32 character
with iconv. Converted utf32 character is finally used as a parameter
of FT_Get_Char_Index.