[HarfBuzz] 'calt' in Indic shaper

[John Hudson]

Dear Jonathan,

> Yes, all this arose as a result of running comparisons (harfbuzz vs
> uniscribe) with Nirmala.

In that case I can safely say that Harfbuzz is doing what I consider the 
right thing, and if the result looks wrong it is because there is a bug 
in the font lookups. Since I couldn't test the lookups in any real world 
shaping engines at that time, it doesn't surprise me that there are bugs.

Luckily, this has come at just the right time, as I'm fixing Nirmala UI 
bugs at the moment.

> For examples of the Gujarati issue in Nirmala (when harfbuzz applies
> 'calt', see
>
> http://people.mozilla.org/~jkew/hb-test/testcase-view.html?file=out/fonts/win8/Nirmala.ttf/gu.txt.ot-vs-uniscribe#G_0_866

Now fixed.

> and many of the other testcases in the list there.
>
> And regarding the Devanagari line-extender, again inserted by 'calt' as
> I understand it, see examples such as
>
> http://people.mozilla.org/~jkew/hb-test/testcase-view.html?file=out/fonts/win8/Nirmala.ttf/hi.txt.ot-vs-uniscribe#L_1_805

That and some related ones really surprise me, because these head line 
extensions should only be happening when the second letter is carrying a 
left-leaning vowel sign. Possibly I'm missing part of a context 
statement, and hopefully I can fix most of these with a simple change in 
the lookups.


J.



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