I've messed up a lot of stuff recently, different parts of the
shaping process are stumbling on eachother's toes because
manually tracking what's in which buffer var is hard.  I'm
going to add some internal API to track those such that mistakes
are discovered as soon as they are introduced.

This frees 16bits in the glyph_info struct during the ot_layout process.
We can use the freed space in the shapers now.

Fallback normalization is complete and working now!

Add tests.

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

Lets not worry about performance for now...

This produces more accurate cluster mappings.  Cluster mappings are
minimal now.  Combining marks get their own cluster value most of
the time.

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.