Returns an object of type glyph-subst-table.
list shall contain a list of pairs of glyph-ids.
In the resulting glyph-subst-table, the substitution for the
first member of each pair is the second member.
The substitution for any glyph-id that does not occur as the first member
of a pair is itself.
If a glyph-id occurs as the first member of more than one pair,
then the substitution for that glyph-id is the second member
of the first pair that has that glyph-id as its first member.
(glyph-subst glyph-subst-table glyph-id)
Returns the glyph-id that substitutes for glyph-id in the