Bring GX point interpolation further into standards compliance

Bring GX point interpolation further into standards compliance

* src/sfnt.c (sfnt_infer_deltas_2): New function; factor much of
sfnt_infer_deltas_1 into this function, then modify its
treatment of untouched points positioned at their reference
points to align with standard GX treatment.
(sfnt_infer_deltas_1): Remove all code not concerning anchor
point discovery.
(main): Adjust tests.

(cherry picked from commit f80f1b23bfd277a9db0ee6961a3d9f09c4cba219)