Part of https://github.com/harfbuzz/harfbuzz/issues/1553

Now that we have 6daf45e0, revert cryptic hacks...

This reverts commit abd81ed4.
This reverts commit 9c6921c0.
This reverts commit d39760ca.
This reverts commit fedd8e6c.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1374

hb-ot-layout-gsubgpos.hh:1707: error: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second:
...

Fixes https://github.com/harfbuzz/harfbuzz/issues/1410

Follow up on b4c61130

Fixes https://github.com/harfbuzz/harfbuzz/issues/1374

Finally:  Fixes https://github.com/harfbuzz/harfbuzz/issues/1356

Test case:
$ ./hb-shape GeezaPro.ttc -u U+0628,U+064A,U+064E,U+0651,U+0629
[u0629.final.tehMarbuta=4+713|u064e_u0651.shaddaFatha=1@0,-200+0|u064a.medial.yeh=1+656|u0628.initial.beh=0+656]

The mark positioning (kern table CrossStream kerning) only works if deleted
glyph (as result of ligation) is still in stream and pushed through the
state machine.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1365

Show up with gcc -O0.

There's a few more but those are functions that need to be filled in.

Maybe this is a lost battle...

From bungeman.

Fixes https://github.com/harfbuzz/harfbuzz/pull/1314

Not sure how can happen, but does...

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

Tested with Kannada MN:

$ HB_OPTIONS=aat ./hb-shape Kannada\ MN.ttc -u 0CCD,0C95,0CD6
[kn_ka.vattu=0+230|kn_ai_length_mark=1@326,0+607]

Fix a bug in CBDT sanitize, and redundant check in avar.

There might be a better way to do this, but I couldn't find...