bug#64420: string-width of … is 2 in CJK environments

[Dmitry Gutov]

Hi! This was reported to company-mode 
(https://github.com/company-mode/company-mode/issues/1388), as a 
scenario that makes the overlay-based completion popup misrender because 
the columns are not computed right when that char is present.

To repro:

  (set-language-environment "Chinese-BIG5")
  (string-width "…") ;; => 2

In the default language environment its width is reported to be 1.

This doesn't seem to make sense because it's rendered one column wide 
either way.