dunai-examples: Upgrade example to use SDL2. Refs ivanperez-keera#446

SDL 1 is deprecated, and no longer maintained. To keep the example
working with more modern versions of packages in the Haskell ecosystem,
this commit ports bouncingball to use SDL2.

dunai-examples/bouncingball/BouncingBall.hs

@@ -34,7 +34,7 @@ main = do
 
    reactimate (return ())
               (\_ -> return (0.005, Just ()))
-              (\_ e -> render e renderer >> return False)
+              (\_ e -> render e renderer)
               sf
 
 sf = bouncingBall (100.0 :: Float) 0.0
@@ -65,8 +65,12 @@ fallingBall p0 v0 = proc () -> do
 -- whenS :: (a -> Bool) -> SF a (Yampa.Event a)
 -- whenS p = (((arr p >>> edge) &&& arr id) >>> (arr (uncurry tag)))
 
-render :: RealFrac a => (a, b) -> SDL.Renderer -> IO ()
+render :: RealFrac a => (a, b) -> SDL.Renderer -> IO Bool
 render (p,_) renderer = do
+  events <- SDL.pollEvents
+  let quit = elem SDL.QuitEvent $ map SDL.eventPayload events
+
+  -- Set background.
   let white = SDL.V4 maxBound maxBound maxBound maxBound
   SDL.rendererDrawColor renderer SDL.$= white
   SDL.clear renderer
@@ -80,6 +84,8 @@ render (p,_) renderer = do
 
   threadDelay 1000
 
+  return quit
+
 -- * Auxiliary functions
 
 -- | Integrate a linearly changing input.