Fix CUBICSPLINE interpolation for morph weights.
Fix CUBICSPLINE interpolation for morph weights.

Tested with Polly the robo-hound and ensured that InterpolationTest
did not regress.

Partial fix for #2714.

google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament