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/examples/application.rs

@@ -457,6 +457,7 @@ impl ApplicationHandler for Application {
                 MouseScrollDelta::PixelDelta(px) => {
                     info!("Mouse wheel Pixel Delta: ({},{})", px.x, px.y);
                 },
+                _ => (),
             },
             WindowEvent::KeyboardInput { event, is_synthetic: false, .. } => {
                 let mods = window.modifiers;

winit/examples/ime.rs

@@ -248,6 +248,7 @@ impl App {
                 }
             },
             Ime::Disabled => info!("IME disabled for Window={:?}", surface.window().id()),
+            _ => (),
         }
     }