No practical point in reusing ligature ids.

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

Not tested at all.

Look forward for next character instead of joining to the last character.

Test case: "مرا" rendered using IranNastaliq.

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.

Graphite module not updated.
Bump version to 0.3.

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.