This includes HB_DIRECTION_INVALID and HB_SCRIPT_INVALID.

The INVALID will cause a "guess whatever from the text" in hb_shape().
While it's not ideal, it works better than the previous defaults at
least (HB_DIRECTION_LTR and HB_SCRIPT_COMMON).

Reported on mailing list by Keith Stribley and Khaled Hosny.

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

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

Patch from Jonathan Kew