On X11, avoid false positive key repeats

Instead of a single `bool` indicating that a key press has occured and
no key has been released since then, we store the scancode of the last
pressed key (if it is a key that repeats when held). This fixes a bug
where pressing a new key while one is already held down will be flagged
as a repeat even though it is obviously not a repeat.

src/platform_impl/linux/x11/mod.rs

@@ -289,10 +289,10 @@ impl<T: 'static> EventLoop<T> {
             xkbext,
             kb_state,
             num_touch: 0,
+            held_key_press: None,
             first_touch: None,
             active_window: None,
             is_composing: false,
-            got_key_release: true,
         };
 
         // Register for device hotplug events