Mark extensible enums in winit-core as #[non_exhaustive]

Event, input source, error, data transfer, native key, IME request,
scroll delta and fullscreen enums can now gain variants without a
breaking change. Backends and examples get wildcard arms where they
matched exhaustively; unknown IME requests are reported as
ImeRequestError::NotSupported and unknown fullscreen modes behave
like Fullscreen::Borderless(None).

winit-x11/src/util/cursor.rs

@@ -199,7 +199,7 @@ impl CustomCursor {
     ) -> Result<CustomCursor, RequestError> {
         let mut cursor = match cursor {
             CustomCursorSource::Image(cursor_image) => cursor_image,
-            CustomCursorSource::Animation { .. } | CustomCursorSource::Url { .. } => {
+            _ => {
                 return Err(NotSupportedError::new("unsupported cursor kind").into());
             },
         };

winit-x11/src/window.rs

@@ -1105,9 +1105,7 @@ impl UnownedWindow {
                             let monitor = monitor.cast_ref::<X11MonitorHandle>().unwrap();
                             (Cow::Borrowed(monitor), None)
                         },
-                        Fullscreen::Borderless(None) => {
-                            (Cow::Owned(self.shared_state_lock().last_monitor.clone()), None)
-                        },
+                        _ => (Cow::Owned(self.shared_state_lock().last_monitor.clone()), None),
                     };
 
                 // Don't set fullscreen on an invalid dummy monitor handle
@@ -2144,6 +2142,7 @@ impl UnownedWindow {
                 self.set_ime_allowed(false);
                 return Ok(());
             },
+            _ => return Err(ImeRequestError::NotSupported),
         };
 
         if let Some((position, size)) = state.cursor_area {