[XeTeX] Incorrect Bengali ZWJ behavior and v2 script spec

[Ian-Mathew Hornburg]

Hello, and thanks to Khaled and contributors for their great work on this
project! I do work in Indic scripts, and XeTeX has been immensely helpful
in setting them.

I may have identified two possible bugs in 0.9999.0: the release notes
indicate that the version-2 OpenType Indic script tags are now supported,
and I’ve been testing various Bengali-script fonts with the git version of
XeTeX and a current install of TeX Live 2012 to check them for correct
shaping behavior. I’ve posted a MWE reproducing some examples from the
Microsoft standard here: [http://pastebin.com/mgAX8c7U].

Microsoft ships two Bengali fonts (Vrinda and Shonar Bangla; both v6.80)
with Windows 8 that support both the older (beng) and newer (bng2) Bengali
script specs. (Older versions of each are shipped with Windows 7 and other
Microsoft products.) The fonts behave correctly when using the beng script
feature, with the exception of a particular ZWJ sequence: the Microsoft
spec [https://www.microsoft.com/typography/OpenTypeDev/bengali/intro.htm]
says that the sequence of consonant-hasant-ZWJ-consonant should prevent a
ligature of the two consonants, then render a half-form of the first
consonant. XeTeX currently fails to suppress the ligature.

However, when passing the bng2 script to the fonts, they both fail to
render correctly at all. I might guess that something’s going wrong because
the font contains both versions of the script tag, and I don’t know what
happens on the HarfBuzz side when selecting which script to use when
shaping a font.

Thanks in advance for any assistance!
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