Added fullscreen_state to Window

* Solves #457

examples/demo.rs

@@ -19,7 +19,6 @@ pub fn main() {
     renderer.set_draw_color(Color::RGB(255, 0, 0));
     renderer.clear();
     renderer.present();
-
     let mut event_pump = sdl_context.event_pump().unwrap();
 
     'running: loop {

src/sdl2/video.rs

@@ -1145,6 +1145,10 @@ impl WindowRef {
         unsafe { ll::SDL_RestoreWindow(self.raw()) }
     }
 
+    pub fn fullscreen_state(&self) -> FullscreenType {
+        FullscreenType::from_window_flags(self.window_flags())
+    }
+
     pub fn set_fullscreen(&mut self, fullscreen_type: FullscreenType)
             -> Result<(), String> {
         unsafe {