Not used yet.  Next commit...

When matching lookups, be smart about default-ignorable characters.
In particular:

Do nothing specific about ZWNJ, but for the other default-ignorables:

If the lookup in question uses the ignorable character in a sequence,
then match it as we used to do.  However, if the sequence match will
fail because the default-ignorable blocked it, try skipping the
ignorable character and continue.

The most immediate thing it means is that if Lam-Alef forms a ligature,
then Lam-ZWJ-Alef will do to.  Finally!

One exception: when matching for GPOS, or for backtrack/lookahead of
GSUB, we ignore ZWNJ too.  That's the right thing to do.

It certainly is possible to build fonts that this feature will result
in undesirable glyphs, but it's hard to think of a real-world case
that that would happen.

This *does* break Indic shaping right now, since Indic Unicode has
specific rules for what ZWJ/ZWNJ mean, and skipping ZWJ is breaking
those rules.  That will be fixed in upcoming commits.

Code cleanup.  No (intended) functional change.

Should NOT change behavior, since first glyph is a match.

Was introduced by 28b9d502.

To be used in upcoming changes.

Fixes 5 Malayalam failures!

MALAYALAM: 1048016 out of 1048334 tests passed. 318 failed (0.0303338%)

Ouch, how did things ever work without this?!  The added test that has a
dot-reph as well as a pre-base reordering Ra perfectly demonstrates the
bug (tested with Nirmala font from Win8 for example).  Testing suggests
that Win8 shaper has the *exact* same bug / behavior that we used to
have.  Odd.

Keep the logic simple, easier to explain to font developers.

This is a followup to 56800027.
Looks like in the Latin shaper, Uniscribe zeroes all Unicode NSM
advances *after* GPOS, not before.  Match that.

Can be tested using DejaVu Sans Mono, since that font has GPOS
rules to zero the mark advances on its own.

Currently broken with Sanskrit 2003 font.

This reverts 167b625d.  It didn't
matter before, but that's going to change with next commit.

This essentially reverts 1d6846db,
but that commit is from way back when.  We should be better
following the spec order now again.

Ouch!

As reported by Jonathan, OS X has setmode() that is something other
than what setmode() is on Win32.  So, limit invocation to Windows
platforms only.

Instead of checking for compiler, check for platform.

The Myanmar shaper now respects the uniscribe-bug-compatibility
option too.

Handles Tai Tham, Cham, and New Tai Lue for now.

Before, we were zeroing advance width of attached marks for
non-Indic scripts, and not doing it for Indic.

We have now three different behaviors, which seem to better
reflect what Uniscribe is doing:

  - For Indic, no explicit zeroing happens whatsoever, which
    is the same as before,

  - For Myanmar, zero advance width of glyphs marked as marks
    *in GDEF*, and do that *before* applying GPOS.  This seems
    to be what the new Win8 Myanmar shaper does,

  - For everything else, zero advance width of glyphs that are
    from General_Category=Mn Unicode characters, and do so
    before applying GPOS.  This seems to be what Uniscribe does
    for Latin at least.

With these changes, positioning of all tests matches for Myanmar,
except for the glitch in Uniscribe not applying 'mark'.  See preivous
commit.

Testing with "clusters" now on par with testing without them.  15
failures both.

Implemented as a hack for now.  Myanmar failures down from 23 to 15.

MYANMAR: 1123868 out of 1123883 tests passed. 15 failed (0.00133466%)

The remaining 15 cases are all where the syllable is wrong according to
the OpenType spec.  We insert dottedcircle.  Uniscribe fails to do that,
but it also fails to reorder the prebase-reordering medial-Ra.  So it
gets it wrong.