Add function `BKE_pose_apply_action_blend()`, which blends a given
Action into current pose. The Action is evaluated at a specified frame,
and the result is applied to the armature's pose.
A blend factor can be given to blend between the current pose and the
one in the Action. Quaternions are interpolated with SLERP; it is
assumed that their FCurves are consecutively stored in the Action.
This function will be used in the upcoming new Pose Library.
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