CoreText doesn't, but no reason we shouldn't do.

Tested with Waseem TTC:

$ hb-shape Waseem.ttc جحخج
[F1Jeem_R2=3@0,180+479|M1Khah_L2_R2=2@0,682+403|M1Hah_L2_R2=1@0,1184+403|I1Jeem_L2=0@0,1184+744]

Also, implement backwards kerning for Format1 in kern and kerx.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1350

Started by reverting https://github.com/harfbuzz/harfbuzz/commit/1f1c85a5

Just a starting point, if we agree even mort can come back.

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...

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11033

SFSNDisplay uses these.  We just apply the default kern without
variations right now.  But at least makes the default kern work.

It has unbounded size...

Fixes https://oss-fuzz.com/v2/testcase-detail/5718889451749376

Fixes https://oss-fuzz.com/v2/testcase-detail/5650286710882304

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=10948

A "&" was missing.  Go back to using pointers that are less error-prone.

Untested.

This concludes kerx table support!

Untested.

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]

Doesn't apply actions yet.