function getViewZNode( builder ) {

	let viewZ;

	const getViewZ = builder.context.getViewZ;

	if ( getViewZ !== undefined ) {

		viewZ = getViewZ( this );

	}

	return ( viewZ || positionView.z ).negate();

}
Returns a node that represents the `z` coordinate in view space
for the current fragment. It's a different representation of the
default depth value.

This value can be part of a computation that defines how the fog
density increases when moving away from the camera.

@param {NodeBuilder} builder - The current node builder.
@return {Node} The viewZ node.