As requested by Jonathan Kew.

We need to devise a better mechanism to choose which scripts to
pass through the Indic shaper.  Moreover, currently we are storing
data for some scripts in the Indic shaper that are not even going
through that shaper.  Need to find a better way...

From:
https://bugzilla.mozilla.org/show_bug.cgi?id=662055

Makes number of failures against Uniscribe with hi_IN dictionary from
OO.o to go down from 6334 to 4290.  Not bad for a one-line change!

Mozilla Bug 729626 - ASAN: heap-buffer-overflow HTML

The OSAtomicAdd32Barrier operator returns the new value, we want the
old value.

So, apparently there's no atomic int 'get' method on Apple.  You have to
add(0) to get.  And that's not const-friendly.  So switch inert-object
checking to a non-atomic get.  This, however, is safe, and a negligible
performance boost too.

Apparently it can't be implemented on OS X.  We weren't using it anyway.

By defining HB_NO_MT.

Also, only warn once per missing MT feature support.

Mozilla Bug 666661 - gfx/harfbuzz/src/hb-prive.h - compiler warnings on mac

Patch from Jonathan Kew.

Bug 662055 - advanced Hebrew diacritics are shown correctly only in
particular order.

Patch from Jonathan Kew.

Bug 662055 - advanced Hebrew diacritics are shown correctly only in particular order

Not that useful right now as we don't escape < and >.  Perhaps
another tool can be added to convert the ANSI output to HTML.

Patch and description from Jonathan Kew:

It turns out that some legacy Thai fonts provide OpenType substitution
features to implement mark positioning, but (incorrectly) put those
features/lookups under the 'latn' script tag instead of using 'thai' (or
possibly 'DFLT'). See
https://bugzilla.mozilla.org/show_bug.cgi?id=719366 for an example and
more detailed description.

Although this is really a font bug, I suggest that we could improve the
rendering of such fonts by looking for the 'latn' as a fallback if
neither the requested script nor "default" is found in
hb_ot_layout_table_choose_script. Suggested patch against harfbuzz
master is attached.

This does _not_ affect the other kind of legacy Thai font, where custom
code to support vendor-specific PUA codepoints would be needed. I'm not
keen to go down that path; IMO, such fonts should be ruthlessly stamped
out in favour of standards-based solutions. :)

JK

This means the --features indices also refer to char position
instead of byte position now.  Same for cluster values reported
by hb-shape.

Will add an option for byte indices later.