[subset] Add HB_SUBSET_FLAGS_IFTB_REQUIREMENTS flag. 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 https://github.com/skef/harfbuzz/commit/3ae2fe2084d96f61d8ad1bbe42a6ef4e62ec830a rebased onto head w/ tests added.