I believe Windows 8 disables it, and spec update dated
Jan 2014 also clearly says it's disabled by default:

  http://www.microsoft.com/typography/OpenTypeDev/arabic/intro.htm#features

Tone-mark reordering and improved Old Hangul support

[unicode] Exclude the Jamo Filler characters from Default_Ignorable, as some fonts want these to be visible/spacing glyphs.

[hangul] Reorder Hangul tone mark to beginning of syllable, unless font implements it using a zero-width glyph.

[hangul] Apply the appropriate *jmo features to decomposed syllables, including Old Hangul sequences that don't have Unicode compositions. Merge clusters in decomposed syllables.

As discussed on
https://github.com/behdad/harfbuzz/pull/10#issuecomment-31442030

Not tested.  Ouch.

Seems to be working now.

Part of https://github.com/behdad/harfbuzz/pull/10

I had tried to expand to fill the blocks, but that sounds wrong in
retrospect.

Now default shaper is truly no-op.

No shaper has more than one behavior re this, so no need for a callback.

Not exhaustively tested, but I think I got the intended logic
right.

The logic can perhaps be simplified.  Maybe we should disabled
normalization with this shaper.  Then again, for now focusing on
correctness.

When seeing U+2044 FRACTION SLASH in the text, find decimal
digits (Unicode General Category Decimal_Number) around it,
and mark the pre-slash digits with 'numr' feature, the post-slash
digits with 'dnom' feature, and the whole sequence with 'frac'
feature.

This beautifully renders fractions with major Windows fonts,
and any other font that implements those features (numr/dnom is
enough for most fonts.)

Not the fastest way to do this, but good enough for a start.

We were throwing away the rtlm feature mask set during
mirroring...

Work around broken automake-1.13 changes.

Fix tests build.

https://bugs.freedesktop.org/show_bug.cgi?id=71353

https://bugzilla.mozilla.org/show_bug.cgi?id=941470