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.
Spacing here is weird? Looks like you're missing a space after `Fullscreen`.