Generalize blended animation to Object3D instances, not just Bone instances.
Store blend weight accumulators in the animationCache object that is created by THREE.Animation.
Remove blend weight accumulators from THREE.Object3D - they have been moved to the animationCache object created by THREE.Animation.
Merge branch 'dev' into refactor_animation_blending
Merge pull request #5084 from ikerr/refactor_animation_blending

Generalize blended animation for Object3D