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.

Commit 6b65a76b.  "end" was becoming
negative.  Was trigerred by Lohit-Kannada 2.5.3 and the sequence:
U+0CB0,U+200D,U+0CBE,U+0CB7,U+0CCD,U+0C9F,U+0CCD,U+0CB0,U+0C97,U+0CB3
Two glyphs were being duplicated.

First, we were abusing OT_VD instead of OT_A.  Fix that
but moving OT_A in the grammar where it belongs (which
is different from what the spec says).

Also, only allow medial consonants after all other
consonants.  This doesn't affect any current character.

Finally, fix Halant attachment in presence of medial
consonants.  Again, this currently doesn't affect any
sequence.

I lied.  There's Gurmukhi U+0A75 which is Consonant_Medial.
Uniscribe allows one of those in each of these positions:
before matras, after matras and before syllable modifiers,
and after syllable modifiers!  We currently just allow
unlimited numbers of it, before matras.

Unicode 6.2.0 Section 16.2 / Figure 16.3 says:

"For backward compatibility, between Arabic characters a ZWJ acts just
like the sequence <ZWJ, ZWNJ, ZWJ>, preventing a ligature from forming
instead of requesting the use of a ligature that would not normally be
used. As a result, there is no plain text mechanism for requesting the
use of a ligature in Arabic text."

As such, we flip internal zwj to zwnj flags for GSUB matching, which
means it will block ligation in all features, unless the font
explicitly matches U+200D glyph.  This doesn't affect joining behavior.