[mark] Only add mark class definitions once
[mark] Only add mark class definitions once

The mark-to-base and mark-to-ligature anchor pairs have different
formats, so putting them all into a set to remove duplicate mark
classes doesn't work. With this change only the mark anchor name is
put in the set.

moyogo/ufo2ft,googlei18n/ufo2ft,jamesgk/ufo2fdk,jamesgk/ufo2ft,googlefonts/ufo2ft