bug#3745: 23.0.95; emacs-23.0.95: unibyte-display-via-language-environment

[Chong Yidong]

> Now `charset_unibyte' is always 0 (i.e. the same as
> `charset_ascii').  So, unibyte->multibyte conversion always
> results in an eight-bit multibyte character.

Looking through the code, I see that the variable `charset_unibyte' is
not initialized properly.  That's the only reason it's 0.  We have to
fix this for sure.

> To fix the above problem, I propose these changes for 23.1
> and the trunk.
>
> (1) Fix all codes accessing charset_unibyte
> (e.g. Funibyte_char_to_multibyte) not to refer to it.

Can we use charset_iso_8859_1 instead of charset_unibyte, or add a line
that says

  charset_unibyte
    = define_charset_internal (...);

in syms_of_charset?

> (2) Setup charset_unibyte correctly in Fset_charset_priority.
>
> (3) Fix x_produce_glyphs to do DECODE_CHAR (charset_unibyte,
>     it->c) instead of unibyte_char_to_multibyte (it->c).

Number 3 is not a trivial change.  IIUC, unibyte_char_to_multibyte is
very fast.  Changing it to use DECODE_CHAR may lead to a performance
hit.