And hb-view --direction argument.

Add variant integers to buffer item types.  More cleanup coming.

We don't cache any metrics internally, so the correct way to add more
glyph metrics items is to add new callbacks for them.  We already have
separeate callbacks for advance vs extents.

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.