bug#39554: 27.0.50; cairo not composing sequences

[James Cloos]

Sequences like 0̸ fail to display composed in master --with-cairo but do
when usin xft.

In a version w/o cairo I get:

Composed with the following character(s) "̸" using this font:
  xft:-unknown-DejaVu Sans Mono-normal-normal-normal-*-22-*-*-*-m-0-iso10646-1
by these glyphs:

and the single char takes up the same width as any ascii letter.

W/ cair i get:

Composed with the following character(s) "̸" using this font:
  ftcrhb:-unknown-DejaVu Sans 
Mono-normal-normal-normal-*-22-*-*-*-m-0-iso10646-1
by these glyphs:
  [0 1 48 19 13 1 12 16 0 nil]
  [0 1 824 704 13 0 13 17 1 nil]

and the single char takes twice the expected width, but still works as a
sing;e char.  OTOH, in the *Help* buffer '"̸"' is three separate chars.
Buth with xft '"̸"' displays with the slash overlaying the first ".
As it should.

The ftcrhb: code needs to display the combining chars over the base
chars like the earlier code does.

-JimC
-- 
James Cloos <cloos@jhcloos.com>         OpenPGP: 0x997A9F17ED7DAEA6