For GPOS pair positioning format 1 the description of ValueRecords
in the OpenType specification (1.8.2, from today) is wrong – the
offset has to be taken from the parent structure; in this case the
`PairSet' table.
* src/otvalid/otvgpos.c (otv_PairSet_validate): Set `extra3'.
(otv_PairPos_validate): Adjust.