[SDL] SDL_WINDOW_ALLOW_HIGHDPI only for OpenGl rendering?

[Dominus]

For Exult (exult.sf.net) I tried adding SDL_WINDOW_ALLOW_HIGHDPI as a flag in SDL_CreateWindow but it won't "take" neither in OS X (I added the plist entry in the app bundle) nor on iOS.
We are using surface to render so I wonder if that's the fault.