gdiplus uses windowscodecs for image file loading and saving, so it
probably doesn't matter much which one you use.
After talk with Nikolay, I eventually choosed to stick with WIC mainly because windowscodecs does not have startup/shutdown functions and thus more light on side effects than gdiplus. The test in this patch series also shows that Windows (probably) uses WIC for image conversion, not gdiplus.
Further testing revealed that invoking WIC for image conversion does not actually load (delay-imported) gdiplus on both Windows and Wine.