gltfio: Fix animation by clamping the per-channel interpolant.
gltfio: Fix animation by clamping the per-channel interpolant.

Tested with the model in #3124 as well as Fox, BusterDrone, CesiumMan,
InterpolationTest, and VC.

Fixes #3124.

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