fix(gltf2): preserve interpolation type and CubicSpline tangents
The glTF2 importer previously ignored the mInterpolation field and discarded
CubicSpline tangent data, leading to incorrect animation playback.
- Implement MapInterpolation() to map glTF2 interpolation types to Assimp.
- Set mInterpolation for all aiVectorKey and aiQuatKey instances.
- For CUBICSPLINE samplers, store [in-tangent, value, out-tangent] triplets
(N x 3 keys) instead of discarding tangents.
- Fixes rendering consistency for InterpolationTest.glb where STEP, LINEAR,
and CUBICSPLINE rows previously rendered identically.
Affected channels: mPositionKeys, mRotationKeys, mScalingKeys.
Co-authored-by: Kim Kulling <kimkulling@users.noreply.github.com>
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