fix(wayland): released keys detection (#378)

On Wayland display `update` function handles key events and then
saves current key state to `keys_prev`. As the resut when we exit from
`update/update_with_buffer` and `keys_prev` and `keys` are equal, so
`get_keys_released` is basically noop.

* Move `key_handler.update()` before handling new key events so that on
exit `keys_prev` contains old data and `key` contains new data (without
overwriting old data).

Issue: #377

Author: woojiq

src/os/posix/wayland.rs

@@ -776,6 +776,8 @@ impl Window {
             self.should_close = true;
         }
 
+        self.key_handler.update();
+
         for event in self.input.iter_keyboard_events() {
             use wayland_client::protocol::wl_keyboard::Event;
 
@@ -951,8 +953,6 @@ impl Window {
                 _ => {}
             }
         }
-
-        self.key_handler.update();
     }
 
     fn handle_key(