[HarfBuzz] Issue regarding chaining contextual substitution

[Dohyun Kim]

Hi,

With hb-view, I have tried a font which has chaining contextual
substitution feature (gsub lookup type 6).
It turned out that the rendering of harfbuzz-ng was not satisfactory.

Command for test:
./hb-view --font-size=48  ~/.fonts/HANDotum-LVT.ttf
"가나갈날ᄀᆞᄂᆞᄀᆞᆯᄂᆞᆯ" > ganada.png

Font tested:
http://ftp.ktug.or.kr/KTUG/hcr-lvt/HANDotum-LVT.ttf  (it's huge: 13M)

Result of harfbuzz-ng (above) compared to ICU library (actually
openoffice; below):
http://people.ktug.or.kr/~nomos/mine/screenshot_harfbuzz_openoffice.png

Yes, I know that harfbuzz-ng does not provide Hangul Jamo shaping
(ljmo, vjmo, tjmo) yet.
However, this font has "calt" feature which is equivalent to ljmo + vjmo + tjmo.
It seems that harfbuzz has invoked this calt feature for rendering;
and the result was good to a large extent.

As shown in the picture, however, last two Korean syllables ("ᄀᆞᆯᄂᆞᆯ")
are not rendered correctly.
This makes me wonder whether it is a bug of harfbuzz-ng or not.

-- 
Dohyun Kim
College of Law, Dongguk University
Seoul, Republic of Korea