F.lux has nothing to do with GPU or shader. Its an application which tweeks display temperature controls. All modern OS provides such interface, on Linux, its x server extension, on Windows its GDI interface. So, its an application that uses the OS interface, that doesn't mean that it has to be made part of OS. At OS level, you provide functionality of underlying hardware but don't decide policies or features. f.lux is a feature not a functionality to display.