static makeClipAdditive( targetClip, referenceFrame = 0, referenceClip = targetClip, fps = 30 ) {

		return makeClipAdditive( targetClip, referenceFrame, referenceClip, fps );

	}
Converts the keyframes of the given animation clip to an additive format.

@static
@param {AnimationClip} targetClip - The clip to make additive.
@param {number} [referenceFrame=0] - The reference frame.
@param {AnimationClip} [referenceClip=targetClip] - The reference clip.
@param {number} [fps=30] - The FPS.
@return {AnimationClip} The updated clip which is now additive.