Normally if you want to, say, conditionally prevent a 'pref', you
would use blocking contextual matching.  Some designers instead
form the 'pref' form, then undo it in context.  To detect that
we now also remember glyphs that went through MultipleSubst.

In the only place that this is used, Uniscribe seems to only care
about the "last" transformation between Ligature and Multiple
substitions.  Ie. if you ligate, expand, and ligate again, it
moves the pref, but if you ligate and expand it doesn't.  That's
why we clear the MULTIPLIED bit when setting LIGATED.

Micro-test added.  Test: U+0D2F,0D4D,0D30 with font from:

[1]
https://code.google.com/a/google.com/p/noto-alpha/issues/detail?id=186#c29

Roboto was hitting this.  FreeType also has pretty much the
same code for this, in ttcmap.c:tt_cmap4_validate():

    /* in certain fonts, the `length' field is invalid and goes */
    /* out of bound.  We try to correct this here...            */
    if ( table + length > valid->limit )
    {
      if ( valid->level >= FT_VALIDATE_TIGHT )
        FT_INVALID_TOO_SHORT;

      length = (FT_UInt)( valid->limit - table );
    }

Can happen either in broken fonts, or as a result of sanitize().

Bug 77800 - cppcheck reports

Apparently they can intermix with other OT_A.

Test: U+0915,A8E2,1CD0

Patch from Martin Hosken.  I expect this to fix the following bugs:

https://bugs.freedesktop.org/show_bug.cgi?id=75076
https://bugzilla.gnome.org/show_bug.cgi?id=723582
https://bugzilla.redhat.com/show_bug.cgi?id=998812

Tests: U+0967,0951 U+0031,093F

Only allow it on DOTTED CIRCLE.  No effect on test numbers.

Test: U+0930,094D,00A0

No functional change intended.

No functional change.

Unicode 6.x IndicSyllableCategory categorizes them as
placeholders, but they can subjoin.

No functional change.

In fact, the previous grammar was ambigious.  No functional
change.

In anticipation of adding more characters to that class of clusters.

It's spacing, but otherwise the same as the other ones.

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