Note: If we do that, we should adapt the packages openimageio, kconfig, kidletime, kitemviews, kplotting, kwidgetsaddons, kcrash, kimageformats, kdeclarative because these currently invoke Xvfb which would be later unnecessary (but harmless, I think).
Not sure why kwidgetsaddons does both set QT_QPA_PLATFORM=offscreen AND invoke Xvfb... why?
