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.
Why `_mainWindowType`? Why not just `_windowType`?