Emulator poll for SDL events

The emulator was ridiculously slow without this change.

Imagine dragging your mouse 100px over the emulator, you would queue
up 100 `MouseMotion` events to be processed.

These SDL events were previously being waited on (ONE AT A TIME) in
the main thread then queued for gesture recognition. The main thread
would then immediately block for 20ms on a gesture which wasn't ready
yet. This would process each event, taking 20ms each, until a gesture
was finally fired.

Instead of waiting for single SDL events, we should send everything we
have straight off to gesture recognition. Then we can wait for the
gesture to be processed.

This shouldn't consume any more CPU since we still have a 20ms wait
for gestures. We've just changed the app to be eager to process
events and happy to wait for gestures.

Fixes #225

crates/emulator/src/main.rs

@@ -270,7 +270,7 @@ fn main() -> Result<(), Error> {
 
     'outer: loop {
         let mut event_pump = sdl_context.event_pump().unwrap();
-        if let Some(sdl_evt) = event_pump.wait_event_timeout(20) {
+        while let Some(sdl_evt) = event_pump.poll_event() {
             match sdl_evt {
                 SdlEvent::Quit { .. } |
                 SdlEvent::KeyDown { keycode: Some(Keycode::Escape), keymod: Mod::NOMOD, .. } => {