landing harfbuzz branch (Re: Status of multicolor fonts?)

[mituharu]

> The general idea is to use HarfBuzz as a font backend which supports
> layout of complex scripts, as a replacement for libm17n-flt on
> GNU/Linux and as a replacement for Uniscribe on MS-Windows.  The
> current code on the branch uses compile-time CPP directives to select
> HarfBuzz as the shaper for xftfont and other font back-ends, but I
> would like to make it a separate font backend instead, so that people
> could choose which one to use.  I also think not every existing font
> backend needs to be able to use HarfBuzz, only those which support OTF
> and TTF font features should.

The attached patch is against the harfbuzz branch, and it adds
`xfthb' and `ftcrhb' backends that use HarfBuzz for shaping.  I
also tried adding the `mac-cthb' backend, but it is not included
in the patch.

With the `ftcrhb' font backend, one can now display regional flag
emojis:

(make-frame '((font-backend ftcrhb)))
(set-fontset-font t '(#x1F1E6 . #x1F1FF) (font-spec :family "Noto Color
Emoji"))
(set-char-table-range composition-function-table '(#x1F1E6 . #x1F1FF)
                      '([".[\x1F1E6-\x1F1FF]" 0 font-shape-gstring]))
(insert (+ ?j (- #x1F1E6 ?a)) (+ ?p (- #x1F1E6 ?a)))

                                     YAMAMOTO Mitsuharu
                                address@hidden

hb-font-backends.diff
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