Untested.

This, enables Format2 for kern table, which was disabled before.

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

Not used in any fonts.  Not well-specified when mixing kerning with
Cross-Stream positioning.

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]

Solves a mystery or two.  I'm fairly confident this is what CoreText does now.

Getting closer.  So many open questions still...