We haven't been keeping this updated.  So, while we don't expose the
symbols in the headers if HB_DISABLE_DEPRECATED is defined, we still
always build them.

This is how Uniscribe does it.  So, adjust.  This is only relevant
to fonts that apply cursive positioning from a contextual lookup.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1181

Enabled if script tag 'Qaag' is passed to HarfBuzz.  Disables mark
advance-zeroing and fallback mark-positioning.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1162

Fixes https://github.com/harfbuzz/harfbuzz/issues/539

Unfortunately our static asserts (DEFINE_SIZE_STATIC) don't actually
fail when used in templates, thanks to SFINAE.  Le sighs.

Probably fixes https://oss-fuzz.com/v2/testcase-detail/5740171484463104

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=10897

From Lorna Evans: "That was a new character added to Unicode 11.0"

See https://github.com/harfbuzz/harfbuzz/pull/730#issuecomment-428277800

Thanks to David Corbett who revamped our script and language processing
and implemented full BCP 47 support.

https://github.com/harfbuzz/harfbuzz/pull/730

New API:
+hb_ot_layout_table_select_script()
+hb_ot_layout_script_select_language()
+HB_OT_MAX_TAGS_PER_SCRIPT
+HB_OT_MAX_TAGS_PER_LANGUAGE
+hb_ot_tags_from_script_and_language()
+hb_ot_tags_to_script_and_language()

Deprecated API:
-hb_ot_layout_table_choose_script()
-hb_ot_layout_script_find_language()
-hb_ot_tags_from_script()
-hb_ot_tag_from_language()

No script has 3 tags yet, but the plan is for the Indic scripts to each
get a third tag someday.

This results in a tenfold speed-up for the common case of tags that are
not complex, in the sense of `hb_ot_tags_from_complex_language`.

OpenType only officially maps four ISO 639 codes to Quechua languages,
but prior versions of HarfBuzz also mapped qu to 'QUZ '. Because qu is a
macrolanguage, the mapping now applies to all individual Quechua
languages. OpenType calls 'QUZ ' "Quechua", but it really corresponds to
Cusco Quechua, so the individual Quechua languages should not all
necessarily be mapped to it.

The font supports the deprecated tag 'DHV ' instead of 'DIV '. dv is
mapped to 'DIV ' and 'DHV ', in that order. The test specifies
`--language=dv`, demonstrating that if a font does not support the first
OpenType tag mapped to a BCP 47 tag, it will fall back to the next tag.

If the second subtag of a BCP 47 tag is three letters long, it denotes
an extended language. The tag converter ignores the language subtag and
uses the extended language instead.

There are some grandfathered exceptions, which are handled earlier.

The new script, gen-tag-table.py, generates `ot_languages` automatically
from the [OpenType language system tag registry][ot] and the [IANA
Language Subtag Registry][bcp47] with some manual modifications. If an
OpenType tag maps to a BCP 47 macrolanguage, all the macrolanguage's
individual languages are mapped to the same OpenType tag, except for
individual languages with their own OpenType mappings. Deprecated
BCP 47 tags are canonicalized.

[ot]: https://docs.microsoft.com/en-us/typography/opentype/spec/languagetags
[bcp47]: https://www.iana.org/assignments/language-subtag-registry/language-subtag-registry

Some OpenType tags correspond to multiple ISO 639 codes. The mapping
from ISO 639 codes lists OpenType tags in priority order, such that more
specific or more likely tags appear first.

Some OpenType tags have no corresponding ISO 639 code in the registry so
their mappings use BCP 47 subtags besides the language. For example, any
BCP 47 tag with a fonipa variant subtag is mapped to 'IPPH', and 'IPPH'
is mapped back to und-fonipa.

Other OpenType tags have no corresponding ISO 639 code because it is not
clear what they are for. HarfBuzz just ignores these tags.

One such ignored tag is 'ZHP ' (Chinese Phonetic). It probably means
zh-Latn. However, it is used in Microsoft JhengHei and Microsoft YaHei
with the script tag 'hani', implying that it is not a romanization
scheme after all. It would be simple enough to add this mapping to
gen-tag-table.py once a definitive mapping is determined.

The manual modifications are mainly either obvious mappings that the
OpenType registry omits or mappings for compatibility with previous
versions of HarfBuzz. Some of the old mappings were discarded, though,
for homophonous language names. For example, OpenType maps 'KUI ' to
kxu; previous versions of HarfBuzz also mapped it to kvd, because kvd
and kxu both happen to be called "Kui".

gen-tag-table.py also generates a function to convert multi-subtag tags
like el-polyton and zh-HK to OpenType tags, replacing `ot_languages_zh`
and the hard-coded list of special cases in `hb_ot_tags_from_language`.
It also generates a function to convert OpenType tags to BCP 47,
replacing the hard-coded list of special cases in
`hb_ot_tag_to_language`.

`hb_ot_tags` replaces `hb_ot_tags_from_script` and
`hb_ot_tag_from_language`.

`hb_ot_layout_table_select_script` replaces
`hb_ot_layout_table_choose_script`.

`hb_ot_layout_script_select_language` replaces
`hb_ot_layout_script_find_language`.

The old hb-ot-tag.cc functions, `hb_ot_tags_from_script` and
`hb_ot_tag_from_language`, are now wrappers around a new function:
`hb_ot_tags`. It converts a script and a language to arrays of script
tags and language tags. This will make it easier to add new script tags
to scripts, like 'dev3'. It also allows for language fallback chains;
nothing produces more than one language yet though.

Where the old functions return the default tags 'DFLT' and 'dflt',
`hb_ot_tags` returns an empty array. The caller is responsible for
using the default tag in that case.

The new function also adds a new private use subtag syntax for script
overrides: "x-hbscabcd" requests a script tag of 'abcd'.

The old hb-ot-layout.cc functions,`hb_ot_layout_table_choose_script` and
`hb_ot_layout_script_find_language` are now wrappers around the new
functions `hb_ot_layout_table_select_script` and
`hb_ot_layout_script_select_language`. They are essentially the same as
the old ones plus a tag count parameter.

Closes #495.

`hb_language_from_string` accepts not only ISO 639 but also BCP 47. Not
all ISO 639 codes are valid BCP 47 tags but the function does not accept
overlong language subtags anyway.

Ned tells me this is what Apple does.

This concludes trak, as well as AAT shaping support!

Works beautifully!  Test coming.