UMovieSceneAnimMixerSystem::PushForceRootBoneDestinationScope

Force-root-bone scope: while the counter is non-zero, normal (non-bus, non-bake-target) mixer programs flip bForceRootBoneDestination on their [FAnimNextStoreRootTransformTask](API\Plugins\MovieSceneAnimMixer\FAnimNextStoreRootTransformTask).