Originally we fixed those in 79d1007a.
However, fonts like MongolianWhite don't have GDEF, but have IgnoreMarks
in their LigatureSubstitute init/etc features.  We were synthesizing a
GDEF class of mark for Mongolian Variation Selectors and as such the
ligature lookups where not matching.  Uniscribe doesn't do that.

I tried with more sophisticated fixes, like, if there is no GDEF and
a lookup-flag mismatch happens, instead of rejecting a match, try
skipping that glyph.  That surely produces some interesting behavior,
but since we don't want to support fonts missing GDEF more than we have
to, I went for this simpler fix which is to always mark
default-ignorables as base when synthesizing GDEF.

Micro-test added.

Fixes rest of https://bugs.freedesktop.org/show_bug.cgi?id=65258