When points are not touched by gvar interpolation deltas, FreeType
gave a slightly different result than Apple's CoreText.
The OpenType working group will update the specification to document
the following behaviour: If the two points with deltas to the `left'
and `right' of the untouched point have the same coordinate, then
the inferred delta for the untouched point should be zero.
* src/truetype/ttgxvar.c (tt_delta_interpolate): Implement new
behaviour.
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