Still finding the exact correct way the masks should be set.

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.