See:
http://lists.freedesktop.org/archives/harfbuzz/2013-April/003101.html

This seems to generate much better, almost-perfect, positioning for
Arabic as well as Latin above marks.

Test with WinXP times.ttf and U+05D9,U+05B5.

Seems to be working just fine!

Lohit-Punjabi has a upem of 769!  We were losing one unit in our
code, and FreeType is losing another one...  Test with U+0A06.
Has an advance of 854 in the font.  We were producing 852.
Now we do 853, which is what FreeType is telling us.

See comments from caveat!  Seems to work fine.

This is useful for Javanese which has an atomically encoded pre-base
reordering Ra which should only be reordered if it was substituted
by the pref feature.

Currently unused.

Impact should be minimal and positive.

Currently unused.

Currently unused.

No functional change.

Both Indic and SEA seem to do it just fine, but SEA is much
simpler.

For scripts without an old/new spec distinction, use zero-context=false.
This changes behavior in Sinhala / Khmer, but doesn't seem to regress.
This will be useful and used in Javanese.

Previously we planted this into the mode used for Khmer.  There's not
really much in common between the two, so separate again.

Similar to 190c8f2b but for
Sinhala.

It's cheap.

Shouldn't change behavior at all, but is faster / more robust.

Whic means these twp are applied per-syllable now.  Apparently
in some Khmer fonts the clig interacts with presentation features.

Test case: U+1781,U+17D2,U+1789,U+17BB,U+17C6 with Mondulkiri-R.ttf
should produce one big ligature.

If a glyph can be both blwf and pref, we were wrongly sorting it
in the post position instead of below position.

Test case: U+09AC,U+09CD,U+09A6 with Lohit-Bengali 2.5.3.