onUpload( callback ) {

		this.onUploadCallback = callback;

		return this;

	}
Sets the given callback function that is executed after the Renderer has transferred
the attribute array data to the GPU. Can be used to perform clean-up operations after
the upload when attribute data are not needed anymore on the CPU side.

@param {Function} callback - The `onUpload()` callback.
@return {BufferAttribute} A reference to this instance.