Implement separated graphics as feature 'ss16'

Implement separated graphics as feature 'ss16'

While Unicode 13 added code points for 6-cell mosaic graphics, it
didn't specify any way to select separated vs contiguous graphics.
This differs fron ZVBI, which used differect code points for separated
and contiguous glyphs.  This means that separated graphics must be a
matter of style, which is consistent with the way that they're
selected by a control character in Teletext.

In OpenType, implement this as a stylistic set, 'ss16', that replaces
the contiguous glyphs with their separated counterparts.  I'm
reserving 'ss14' for the 4-cell equivalent.