The following sed command can be used to update TTX sources for the LookupType
change:
$ sed -i'~' 's/<!-- LookupType=\(.\) -->/<LookupType value="\1"\/>/g' *.ttx
No changes in the compiled binary representation, this just affects XML.
After this change, the XML for empty ValueRecords is identical to the
output from makeotf.
https://github.com/behdad/fonttools/issues/471