A while back I changed code such that Lookup.LookupType is written as a
comment in XML, and ignored when compiling. The LookupType from type
of actual subtables in a lookup were used during compilation instead.
This caused the problem where an empty lookup (one with no subtables)
would lose its lookup types, among other subtle problems.
With this change we revert above behavior, but keep the benefits: if
Lookup.LookupType is different from actual lookup type of the subtables,
compilation raises an exception. Setting LookupType on Lookup object
or in XML is optional now, but written out by default in XML (instead
of as a comment).
This changes XML output for all GSUB/GPOS tables. I'm sorry for the
noise. Please update your sources.
Fixes https://github.com/fonttools/fonttools/issues/789
495c6831e1