function isCubeMapReady( image ) {

	if ( image === null || image === undefined ) return false;

	let count = 0;
	const length = 6;

	for ( let i = 0; i < length; i ++ ) {

		if ( image[ i ] !== undefined ) count ++;

	}

	return count === length;


}
Returns `true` if the given cube map image has been fully loaded.

@private
@param {?Array<(Image|Object)>} [image] - The cube map image.
@return {boolean} Whether the given cube map is ready or not.