bug#26742: Display bug with composed strings

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 03 May 2017 12:50:16 +0900
> From: YAMAMOTO Mitsuharu <mituharu@math.s.chiba-u.ac.jp>
> Cc: 26742@debbugs.gnu.org
> 
> +/* Return glyph string that shares background with glyph string S and
> +   whose `background_width' member has been set.  */
> +
> +static struct glyph_string *
> +glyph_string_containing_background_width (struct glyph_string *s)
> +{
> +  if (s->cmp)
> +    while (s->cmp_from)
> +      s = s->prev;
> +
> +  return s;
> +}

Please also test this with composed characters in R2L text (I don't
remember if s->cmp_from still has the same meaning in that case).

Thanks.