Default features only for now.

We expect buffer to be setup with default positions before GPOS.

The array query APIs now all do:

  - Return the total number of items
  - Take a start_offset, such that individual items can be fetched
  - The _count IN/OUT variable always has number of items written
    into the array upon return of the function

That function doesn't make much sense as multiple features in the table may
have the same tag.

This reverts commit c939f6aff405ca7b10b1f1538f46148bff719fcb.

Wow, IranNastaliq renders perfectly again!