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.

Looks like a roadmap now.

It's tested now.  It works!

Not tested yet.

Not public yet.

This is the object that a separate plan/execute shaping API will use in
between the two stages.

Untested code is indeed buggy code.

Mozilla bug #603352.

Mozilla bug #603346

hb_ot_plan_t may be a better name, donno.

We now build our entire attack plan before jumping in.