[HarfBuzz] Harfbuzz Sinhala (si) script support status update

[Harshula]

Hi Behdad,

I tested the recent Harfbuzz Indic code changes. The correctness of
Sinhala script layout/rendering has improved significantly.

1) Notably, "Bug 45114 - Enable ZWJ in lookups for Sinhala and
Malayalam" (https://bugs.freedesktop.org/show_bug.cgi?id=45114) has been
fixed by the following commit by removing the offending code:

--------------------------------------
commit d1deaa2f5bd028e8076265cba92cffa4fa2834ac
Author: Behdad Esfahbod <behdad at behdad.org>
Date:   Wed May 9 15:04:13 2012 +0200

    Replace zerowidth invisible chars with a zero-advance space glyph

    Like Uniscribe does.
--------------------------------------

2) Once "Bug 50706 - [si] harfbuzz does not render diga kombuva (U+0DDA)
correctly when combined with a
consonant" (https://bugs.freedesktop.org/show_bug.cgi?id=50706) is
fixed, Sinhala script support should be sufficient for public
consumption.

3) Also keep in mind that a Sinhala 'Virama' (Al-Lakuna) does NOT create
implicit conjuncts when surrounded by consonants. In Pango and ICU I had
to change the state table to add a separate al-lakuna column.

cya,
#