* hb_aat_get_feature_settings

Context manager.

Fixes shaping with Apple Chancery on 10.13 again.  In that font,
there was a terminator segment, that was tripping off sanitize().

This reverts commit ae8ed58a.

Apparently this broke Apple Chancery from OS X 10.12 :(.
Investigating...

This reverts commit 15905a2a.

This reverts commit bbdb6edb.

Have not encountered fonts needing this, but same reasoning as
for morx (see previos commit.)

With OS X 10.13 Apple Chancery fails to ligate if we limit each morx
sub-chain to its declared length.  Perhaps their newer compiler does
object-sharing across sub-chains.  Anyway, since that's a valid, if
unspecified, way to compile tables, remove enforcement.

Probably do the same with kern/kerx.

Broke in 8dcc1913

If sanitizer is left with another object, it wouldn't work.

Better fix coming soon.

Much more useful now. :)