We should ensure-direction before doing any complex work.  The only
exception is mirroring that needs to see the original / final direction,
not the native.  Handle that.

Patch from suzuki toshiya.

Patch from Thomas Hunger.

Mandaic was added to Unicode 6.0, but the joining data was not updated.
Draft ArabicShaping.txt from 6.1 includes the joining data for Mandaic.
Use that.

Based on tags proposed by Microsoft for inclusion in OpenType.

The new name matches the Unicode name.  Reported by Jonathan Kew.

Ugly :(.

No practical point in reusing ligature ids.

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