Ugly :(.

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.