When enabled the output subset conforms to the requirements for a base font to be patched by IFTB patches. IFTB is a proposed incremental font transfer patch. This new flag is set as experimental. Currently, setting this flag causes the output subset to also use long offsets for outline data in loca/glyf, gvar, CFF, and CFF2.
This change is a version of 3ae2fe2084 rebased onto head w/ tests added.