* Guard against underflow when adjusting length

With the fuzz-testcase in mozilla bug 1295299, we end up with a recursed lookup that removes 3 items, when `match_positions[idx]` is 0, which results in (unsigned) `end` wrapping to a huge value.

Making `end` a signed int is probably the simplest route to a fix.

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1295299.

* Add testcase for #421.

* [indic] Add support for Grantha marks that may be used in Tamil to the Indic table.

See https://bugzilla.mozilla.org/show_bug.cgi?id=1331339.

Testcase: U+0BA4,U+0BC6,U+1133c,U+0BAA,U+1133c,U+0BC6,U+1133c

* [indic] Add test for Grantha nukta that is allowed in Tamil by ScriptExtensions.txt

[graphite] Fix shaping with varying font sizes

Hopefully fixes Travis faliure?
https://github.com/behdad/harfbuzz/pull/413#issuecomment-278481882
https://travis-ci.org/khaledhosny/harfbuzz/jobs/199785332#L1065

While it’s fine to call memcmp(x, 0, 0) in practice, the C99 standard
explicitly says that this is not allowed: even if the length is zero,
the pointer arguments must be valid.

http://stackoverflow.com/a/16363034

Coverity ID: 141178
Signed-off-by: NPhilip Withnall <withnall@endlessm.com>

Might help with https://github.com/behdad/harfbuzz/issues/399

Coverity IDs: 141039, 141040, 141041

Coverity ID: 141042

Fixes https://github.com/behdad/harfbuzz/issues/408

For completeness.

Fixes https://github.com/behdad/harfbuzz/issues/411

Fixes joined Adlam rendering.

Fixes https://github.com/googlei18n/noto-fonts/issues/828

Unhooked. Untested.

Yay, works!

Not hooked up to hb-ot-font yet.

We're not going to define public macros for tags for tables.
There's little use to those and they are implementation detail.

Untested!

The 'avar' table does not allow random access to axis maps,
so change API to avoid quadratic-time implementation.

Removed -hb_ot_var_normalize_axis_value(), added
+hb_ot_var_normalize_variations() and
+hb_ot_var_normalize_coords() instead.