Set one property of FONT: give property KEY value VAL.
FONT is a font-spec, a font-entity, or a font-object.
If FONT is a font-spec, KEY can be any symbol. But if KEY is the one
accepted by the function `font-spec' (which see), VAL must be what
allowed in `font-spec'.
If FONT is a font-entity or a font-object, KEY must not be the one
accepted by `font-spec'.
(fn FONT PROP VAL)