Make pollEvent not allocate unnecessarily (#178)

Just to summarize the numbers below for future reference:

> let timeToPoll = 11 * 1e-6 -- from criterion
> let bytesPerPoll = 36802144/100000 -- from weigh
> let pollsPerSec = 1/timeToPoll
> let bytesPerSecond = bytesPerPoll * pollsPerSec
> pollsPerSec
90909.09090909091
> printf "%f" (pollsPerSec * bytesPerPoll)
33456494.545454547

So that's 90,909 polls and 33.5MB per second of allocations. That's
consistent with the below findings. The GC count for 100,000
polls (36.8MB) is 35 GCs, so that's roughly 1 GC per 1MB which
corresponds to GHC's default GC settings.

Before:

Case                Allocated  GCs
pollEvents 10           4,632    0
pollEvents 100         38,416    0
pollEvents 1000       369,112    0
pollEvents 10000    3,680,088    3
pollEvents 100000  36,802,144   35

benchmarking pollEvents/orig
time                 11.43 μs   (11.37 μs .. 11.49 μs)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 11.41 μs   (11.37 μs .. 11.48 μs)
std dev              167.3 ns   (122.4 ns .. 274.7 ns)
variance introduced by outliers: 11% (moderately inflated)

After

Case               Allocated  GCs
pollEvents 10          1,400    0
pollEvents 100         1,400    0
pollEvents 1000        1,400    0
pollEvents 10000       1,400    0
pollEvents 100000      1,400    0

benchmarking pollEvents/check
time                 11.05 μs   (11.00 μs .. 11.11 μs)
                     1.000 R²   (0.999 R² .. 1.000 R²)
mean                 11.04 μs   (10.99 μs .. 11.13 μs)
std dev              216.5 ns   (154.8 ns .. 296.8 ns)
variance introduced by outliers: 19% (moderately inflated)

Author: chrisdone

src/SDL/Event.hs

@@ -746,11 +746,18 @@ convertRaw (Raw.UnknownEvent t ts) =
 
 -- | Poll for currently pending events. You can only call this function in the thread that set the video mode.
 pollEvent :: MonadIO m => m (Maybe Event)
-pollEvent = liftIO $ alloca $ \e -> do
-  n <- Raw.pollEvent e
-  if n == 0
-     then return Nothing
-     else fmap Just (peek e >>= convertRaw)
+pollEvent =
+  liftIO $ do
+    n <- Raw.pollEvent nullPtr
+    -- We use NULL first to check if there's an event.
+    if n == 0
+      then return Nothing
+      else alloca $ \e -> do
+             n <- Raw.pollEvent e
+             -- Checking 0 again doesn't hurt and it's good to be safe.
+             if n == 0
+               then return Nothing
+               else fmap Just (peek e >>= convertRaw)
 
 -- | Clear the event queue by polling for all pending events.
 pollEvents :: (Functor m, MonadIO m) => m [Event]