To be used soon.

Fixes http://github.com/behdad/harfbuzz/pull/36

Test with:

hb-view /Library/Fonts/Zapfino.ttf ZapfinoZapfino --features=-dlig[7:] --shaper=coretext

Apparently those functions documented as sometimes returning NULL
actually exercise that right in OS X 10.10 Yosemite.  The scratch
was too small for that.  I *think* I fixed it, but haven't tested
as I don't have Yosemite.

Simplifies hb_in_range() calls as the type can be inferred.
The rest is obsessiveness, I admit.

In an effort to make the algorithm used clear.

Not tested.

This is a higher-priority shaper than default shaper ("ot"), but
only picks up fonts that have AAT "morx"/"mort" table.

Note that for this to work the font face's get_table() implementation
should know how to return the full font blob.

Based on patch from Konstantin Ritt.

Fixes github.com/behdad/harfbuzz/pull/19

CoreText does automatic font fallback (AKA "cascading") for  characters
not supported by the requested font, and provides no way to turn it off,
so detect if the returned run uses a font other than the requested one
and fill in the buffer with .notdef glyphs instead of random indices
glyph from a different font.

Based on patch from Jonathan Kew and data from Apple.

It's not working correctly though, and I suspect I'm hitting a bug in
CoreText.  When I do this:

hb-shape /Library/Fonts/Zapfino.ttf ZapfinoZapfino --shaper coretext \
--features=-liga

I expect both ligatures to turn off, but only the second one does:

[Z_a_p_f_i_n_o=0+2333|Z=7+395|a=8+285|p_f=9+433|i=11+181|n=12+261|o=13+250]

whereas if I disable 'dlig' instead of 'liga', both are turned off.
Smells...

Doesn't resolve conflicting feature settings.

Unlike AAT, looks like with CoreText very few OT features can be mapped.
:(

Patch from Jonathan Kew.

https://bugs.freedesktop.org/show_bug.cgi?id=65544

Oops.  Thanks Khaled for catching this.

Not sure if it's useful, but it was missing.

We not namespace our types, so the hack is not needed anymore.

Add global shape_plan.  Unused so far.

Does not provide Uniscribe-compatible results, but should at least avoid
breaking hb-view due to out-of-order cluster values.

For RTL runs, ensure cluster values are non-increasing (instead of
non-decreasing).

Does not attempt to handle clusters in a Uniscribe- or HarfBuzz-compatible way;
just returns the original string indexes that CT maintains. These may even be
out-of-order in the case of reordrant glyphs.