If offset overflow happens other than between Lookup and SubTable, or within SubTable, then we don't know how to fix it currently. Now we try to put that lookup in an Extension lookup anyway, since that will isolate it and sharing with other lookups won't happen, hopefully avoiding the overflow. This is really a bandaid... What we want is the 99proof branch to be finished and replace our current packing algorithm. Fixes https://github.com/fonttools/fonttools/issues/1296