QIcon::pixmap doesn't know anything about the paint device currently
being used so uses qApp->devicePixelRatio.
This works in systemsettings, but not in kwin and we get a standard
resolution pixmap returned.
We can't manually multiply by painter->device->dpr because then system
settings would get scaled twice.
QIcon::paint solves that in a way that won't break should kwin's
qApp->dpr change. Also it's IMHO cleaner as it does the alignment