Return the value of FONT's property KEY.
FONT is a font-spec, a font-entity, or a font-object.
KEY is any symbol, but these are reserved for specific meanings:
:family, :weight, :slant, :width, :foundry, :adstyle, :registry,
:size, :name, :script, :otf
See the documentation of `font-spec' for their meanings.
In addition, if FONT is a font-entity or a font-object, values of
:script and :otf are different from those of a font-spec as below:
The value of :script may be a list of scripts that are supported by the font.
The value of :otf is a cons (GSUB . GPOS) where GSUB and GPOS are lists
representing the OpenType features supported by the font by this form:
((SCRIPT (LANGSYS FEATURE ...) ...) ...)
SCRIPT, LANGSYS, and FEATURE are all symbols representing OpenType Layout tags. (fn FONT KEY)