This simplifies the code, reduces static data, and makes the design
more extensible to future additions of new scripts.

Previously boolean features turned on the entire feature mask.  This is
wrong if feature is Alternate and user has provided values bigger than one.
Though, I don't think other engines support such corner cases.

Previously if a default global feature was overrided by a non-global
user feature, we were not setting any default mask for the feature,
essentially disabling the feature by default.  Fix that.

hb_ot_plan_t may be a better name, donno.