Return a list of variation glyphs for CHAR in FONT-OBJECT.
Each element of the value is a cons (VARIATION-SELECTOR . GLYPH-ID),
where
VARIATION-SELECTOR is a character code of variation selection
(#xFE00..#xFE0F or #xE0100..#xE01EF)
GLYPH-ID is a glyph code of the corresponding variation glyph.
(fn FONT-OBJECT CHARACTER)