[makeotfexe] 'lookup useExtension' does not work right with contextual substitution lookups

[readroberts]

When a contextual lookup is made into an extension lookup, the actual contextual rules do go in extension subtables. However, the substitution rules that are applied by the contextual rules are accumulated in a separate lookup, whose subtables are NOT made into extension subtables. The problem code has been identified, but it will be a few days before the fix makes into a release. This bug was found by Edgar Walthert, who is the first to push on this boundary.