This is essential as ensure_native_direction uses cluster info that
is set by form_clusters().

Spacing_Mark is the current Unicode long-name for this property value.
The previous name was wrongly carried from glib.

Adjust glib fallback implementation.

The tests are not hooked up for ICU yet.

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().

Add compose() and decompose() unicode funcs.  These implement
pair-wise canonical composition/decomposition.

The glib/icu implementations are lacking for now.  We are adding
API for this to glib, but I cannot find any useful API in ICU.
May end of implementing these in-house.

Changed all unicode_funcs callback names to remove the "_get" part.
Eg, hb_unicode_get_script_func_t is now hb_unicode_script_func_t,
and hb_unicode_get_script() is hb_unicode_script() now.

In preparation for adding more advanced unicode funcs.

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.

I'm too lazy to fix the tests now.

Mozilla Bug 669175 - Slow rendering of text sometimes in this case,
using direction: rtl

Still not much there.

They are outdated with respect to the DejaVu Sans I'm using.
We need to add font version checking to the tests.

One has to run the test with --verbose to see that right now.

Towards a better test runner.

We'd add normalization and decomposition tests later.