function getCacheKey( renderContext ) {

	const { textures, activeCubeFace } = renderContext;

	const values = [ activeCubeFace ];

	for ( const texture of textures ) {

		values.push( texture.id );

	}

	return hashArray( values );

}
Computes a cache key for the given render context. This key
should identify the render target state so it is possible to
configure the correct attachments in the respective backend.

@param {RenderContext} renderContext - The render context.
@return {number} The cache key.