bug#40502: 28.0.50; Hangul Jamo conjoining characters are not composed

[Robert Pluim]

According to Unicode 13 Chapter 18.6, Hangul Jamo characters should be
composed into the corresponding pre-composed Hangul characters. Iʼm
not seeing this, e.g

하 (U+1112 U+1161) should be displayed as 하 (U+D558), but is shown as
two separate glyphs (all using Apple SD Gothic Neo)

(set-char-table-range composition-function-table
                      #x1112
                      (list (vector "..")
                            0
                            'compose-gstring-for-graphic))

does not help.

This is:

In GNU Emacs 28.0.50 (build 127, x86_64-apple-darwin18.7.0, NS appkit-1671.60 
Version 10.14.6 (Build 18G4032))
 of 2020-04-02 built on rpluim-mac

although I see the same in a Cairo+HarfBuzz build on GNU/Linux.