[HarfBuzz] Complex NAA ligatures in Tai Tham

[Theppitak Karoonboonyanan]

I have updated my Lao Tham font in attempt to cover NAA ligatures
with Harfbuzz preprocessing. It seems to work with simple cases with
<NA, vowel AA> next to each other, and even with a subjoined consonant
in between like <WA, NA, SAKOT, LOW TA, vowel AA>.
But it will fail if there is a leading vowel before vowel AA, like in
<HIGH PA, NA, SAKOT, HIGH THA, vowel E, vowel AA>.

The font is here:
  http://linux.thai.net/~thep/esaan-scripts/fonts/Khottabun-hb.ttf

I have attached the text for testing (tham-naa.txt), the result
(tham-naa.png), and the expected result edited with GIMP
(tham-naa-expected.png).

Regards,
--
Theppitak Karoonboonyanan
http://linux.thai.net/~thep/
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