bug#19993: 25.0.50; Unicode fonts defective on Windows

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 7 Mar 2015 23:41:58 -0800
> From: Ilya Zakharevich <ilya@math.berkeley.edu>
> Cc: 19993@debbugs.gnu.org
> 
> The following subrange is “identified”:
> 
>   9       0400 - 04FF          Cyrillic
>           0500 - 052F          Cyrillic Supplement
>           2DE0 - 2DFF          Cyrillic Extended-A
>           A640 - A69F          Cyrillic Extended-B
> 
> What is actually supported:
> 
>   0400 - 04FF    Everything but 04d8,04d9 (Schwa, used in Cyrillic Azeri — but 
> contemporary Azeri is written in Latin) 
>   0500 - 052F    Only 0500 - 0513, 051a - 051d supported
>   2DE0 - 2DFF    None supported (5.1)
>   A640 - A69F    None supported (5.1 and later)

FWIW, on one system I use frequently (a pretty standard Windows 7
machine), the last two blocks are not supported by any font.  Likewise
on another system, an XP with a lot of additional fonts installed.
But if I install GNU Unifont, then Emacs does use it for these blocks.
So somehow we do TRT in some situations.  Maybe these subranges are
just a filter?

> Does it look like a good indication of anything?  I would say no… Do
> you know any other tool looking at this bitmap for choosing which font
> to pick up for a particular character?

I guess by "tool" you mean a technique or algorithm that uses the font
data for selecting the appropriate font?  If so, no, I know very
little in this area, on Windows or elsewhere.  Enumerating all the
glyphs would work, but is probably prohibitively expensive.