I've expanded the number of bits from 16->32 without impacting
the overall struct sizes and reserved 2 bits for super/subscript.
I refer to these as vertical alignment properties for conceptual
consistency with css.
SGR 73, 74, 75 are used to set super, sub and normal vertical alignment.
These are compatible with mintty.
However, mintty just added support for setting both attributes to render in
small caps in 06ac446049
(https://github.com/mintty/mintty/issues/1171)
