Changed the "fullscreen" config variable to an enum

...Which can be either "Fullscreen", "FullscreenDesktop", or "Windowed".
I also udated the win32gl adapter to reflect this.
Also cleaned up some cruft in sdl.d and changed the two deprecated
logFatal() statements in linux.d to fatal()s.
Based on [the SDL docs](https://wiki.libsdl.org/SDL_WindowFlags), it seems like all this does is just use the default desktop resolution. It seems to me that we want `SDL_WINDOW_BORDERLESS` at the native resolution instead. That is, assuming we rename this to `FullscreenWindowed`.