Fixes https://github.com/behdad/harfbuzz/issues/503

Fixes https://github.com/behdad/harfbuzz/issues/516

This should affect mark attachment in vertical text.  I have no font to test,
but this sounds the right order.

Noticed while debugging https://github.com/behdad/harfbuzz/issues/532

Fixes https://github.com/behdad/harfbuzz/issues/552

Fixes https://github.com/behdad/harfbuzz/issues/528

Related to https://github.com/behdad/harfbuzz/issues/528

Apparently a base glyph can also become an attached component of a
ligature if the ligature-forming lookup used IgnoreBase.  This was
being confused with a non-first component of a MultipleSubst and
hence not matched for mark-attachment.  Tweak test to fix.

Fixes https://github.com/behdad/harfbuzz/issues/543

Followup to 8b2c94c4

Allow matching sequences of marks attached to different ligatures,
as supposedly the base of the subsequent marks were already jumped
over.

If two marks want to ligate and they belong to different components of the
same ligature glyph, and said ligature glyph is to be ignored according to
mark-filtering rules, then allow.

Example Burmese senquence:

  U+1004,U+103A,U+1039,U+101B,U+103D,U+102D

Test font provided by Norbert Lindenberg.

Fixes https://github.com/behdad/harfbuzz/issues/545

Mark it as matra below to allow the sequence U+0A15, U+0A51, U+0A47.
Oh well...

Fixes https://github.com/behdad/harfbuzz/issues/524

Fixes https://github.com/behdad/harfbuzz/issues/525

Update to commit c000ebf79c095a7d58cf90090bde5715592c4834
plus this bug-fix: https://github.com/grigorig/ucdn/issues/18

Fixes https://github.com/behdad/harfbuzz/issues/538

This is a temporary hack just to avoid incorrect shaping results, pending proper support
for caching shape-plans with variation coordinates.

* Fix missing xscale in rtl graphite unpacking

* Oops didn't need to mess with yscale

Test with, eg.:
$ LC_ALL=fr_FR.utf-8 ./hb-view NotoSansArabic-VF.ttf بهداد --variations wght=1.2

If buffers are empty, content type should be ignored.

This fixes last of the failing tests: fuzzed.tests.  Green again!

Fixes tests/shaping/tests/cluster.tests

Fixes https://github.com/behdad/harfbuzz/issues/528
"Kannada JIHVAMULIYA and UPADHMANIYA insert dotted circles"

Fixes tests/automatic-fractions.tests

I like to have a mode where CONTAINS_NOTDEF and CONTAINS_DOTTEDCIRCLE are not
returned.  Abused a value of -1 for that.  hb-shape now uses it.  Fixes two
of the six tests failing with --verify in test/shaping/run-tests.sh.

New API.

Change hb_buffer_diff to explicitly cast result of abs to unsigned when
comparing with position_fuzz to avoid unsafe signed/unsigned comparions
warnings on windows.

Based on patch from Jonathan Kew.

Needs more cleaning up and documentation.

New API:
hb_buffer_diff_flags_t
hb_buffer_diff()