Like Uniscribe does.

As reported by Jonathan Kew on the list.

In preparation for Hangul shaper.

According to Peter Constable this is indeed what Uniscribe has been
doing for years.

Mozilla Bug 667166 - wrong shape of letter when it comes at the end of
word in the arabic version of Firefox 5.0

Where causing issues for people with MSVC.

This allows the Indic shaper to request decomposed characters.  This will
handle split matra for free.  Other shapers prefer precomposed
characters.

Instead of always applying those two features before the complex shaper,
let the complex shaper decide whether they should be applied first.

Also add stub for Indic's final_reordering().

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=644184
among others.

Shapers now can request segmented feature application by calling
add_gsub_pause() or add_gpos_pause().  They can also provide a
callback to be called at the pause.  Currently the Arabic shaper
uses pauses to enforce certain feature application.  The Indic
shaper can use the same facility to pause and do reordering in the
callback.

Nothing functional in there yet.

So far, we're parsing IndicSyllabicCategory.txt and IndicMatraCategory.txt
fils from Unicode Character Database and store them in an array to be used
by the shaper.  Also hooked up the shaper, but it does not do anything
right now.

Respectively, separate planner from the actual plan.

So we can liberally use the simple features of C++ that parts of the
codebase is already using.

That better matches OpenType spec.  Note that we enable it for all
Arabic-shaper scripts.  Ie. we enable it by default for Syriac too,
but the SyriacOT spec does not require it.  I think this is a more
useful compromise than special-casing for Arabic script alone.

Add support for classic Mongolian script to the Arabic shaper.

Still work to be done around U+180E MONGOLIAN VOWEL SEPARATOR as it
should not be included in the final glyph stream the same way that
ZWNJ, etc should not appear in the final glyph stream.

But the joining part should be done.

There remains the question of how should the U+18A9 MONGOLIAN LETTER ALI
GALI DAGALGA be handled as it has General Category NSM but a letter
nonetheless.  For now, our generic logic makes this a joining T instead
of joining D as other Mongolian letters are.

Mandaic was added to Unicode 6.0, but the joining data was not updated.
Draft ArabicShaping.txt from 6.1 includes the joining data for Mandaic.
Use that.

Add variant integers to buffer item types.  More cleanup coming.

Previously boolean features turned on the entire feature mask.  This is
wrong if feature is Alternate and user has provided values bigger than one.
Though, I don't think other engines support such corner cases.

It's tested now.  It works!

Not tested yet.

Not hooked up just yet.