A character style's OpenType features took nine tags. OtFeatures now
also keeps ss01-ss20, salt, titl, pnum, lnum, sups and subs (off by
default, one bit each in `extra`): they are accepted by the type
commands, stored with the style and passed to the shaper, so a font
that has them uses them.
The Affinity import test used ss01 as its example of a feature kept but
not rendered; it now uses cv01 for that (character variants still
aren't rendered) and keeps ss01 as one that is.
Fixes#1607
Gert Pellin
bflatastic
Riccardo Balloch