Up to two of them come after all OT_A characters.

According to Roozbeh, there are valid combinations in Unicode
proposals for up to three.  Previously we were allowing up to two.

Also adjust U+0953,0954 handling.

Sinhala and Telugu use "explicit" reph.  That is, the reph is formed by
a Ra,H,ZWJ sequence.  Previously, upon detecting this sequence, we were
checking checking whether the 'rphf' feature applies to the first two
glyphs of the sequence.  This is how the Microsoft fonts are designed.
However, testing with Noto shows that apparently Uniscribe also forms
the reph if the lookup ligates all three glyphs.  So, try both
sequences.

Doesn't affect test results for Sinhala or Telugu.

https://code.google.com/a/google.com/p/noto-alpha/issues/detail?id=232

Fixes https://github.com/behdad/harfbuzz/pull/40

The grammar in the OT spec, and the existing Windows implementation
seem to be confused around where to allow Asat around the medial
consonants.

The previous grammar for medial group was allowing an Asat after
the medial group only if there was a medial Wa or Ha, but not if
there was only a medial Ya.  This doesn't make sense to me and
sounds reversed, as both medial Wa and Ha are below marks while
Asat is an above mark.  An Asat can come before the medial group
already (in fact, multiple ones can.  Why?!).  The medial Ya
however is a spacing mark and according to Roozbeh it's valid
to want an Asat on the medial Ya instead of the base, so it looks
to me like we want to allow an Asat after the medial group if
there *was* a Ya but not if there wasn't any.  Not wanting to
produce dotted-circle where Windows is not, this commit changes
the grammar to allow one Asat after the medial group no matter
what comes in the group.

Test: U+1002,103A,103B vs U+1002,103B,103A

One of them currently produces dotted-circle.  Fix and detailed
message coming.

Before we were just relying on the compiler inlining them and not
leaving a trace in our public API.  Try to fix.  Hopefully not
breaking anyone's build.

Based on suggestion from Andrew Glass.

Test: U+0F40,0FC6,0F83

Currently fails.

commit b5a0f69e
Author: Behdad Esfahbod <behdad@behdad.org>
Date:   Thu Oct 17 18:04:23 2013 +0200

    [indic] Pass zero-context=false to would_substitute for newer scripts

    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.

The *intention* was to change zero-context from true to false for scripts that
don't have old-vs-new specs.  However, checking the code, looks like we
essentially change zero-context to always be true; ie. we only changed things
for old-spec, and we broke them.  That's what causes this bug:

  https://bugs.freedesktop.org/show_bug.cgi?id=76705

The root of the bug is here:

  /* Use zero-context would_substitute() matching for new-spec of the main
   * Indic scripts, but not for old-spec or scripts with one spec only. */
  bool zero_context = indic_plan->config->has_old_spec || !indic_plan->is_old_spec;

Note that is_old_spec itself is:

  indic_plan->is_old_spec = indic_plan->config->has_old_spec && ((plan->map.chosen_script[0] & 0x000000FF) != '2');

It's easy to show that zero_context is now always true.  What we really meant was:

  bool zero_context = indic_plan->config->has_old_spec && !indic_plan->is_old_spec;

Ie, "&&" instead of "||".  We made this change supposedly to make Javanese
work.  But apparently we got it working regardless!  So I'm going to fix this
to only change the logic for old-spec and not touch other cases.

Disables any cluster-merging.  Added for testing purposes while
we investigate what kind of API to add for this.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=75384

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=74491

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=74490

As requested by Martin Hosken on the list.

Many GCC versions don't define __arm64__