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-core/src/monitor.rs

@@ -183,6 +183,7 @@ impl fmt::Display for VideoMode {
 
 /// Fullscreen modes.
 #[derive(Clone, Debug, PartialEq, Eq)]
+#[non_exhaustive]
 pub enum Fullscreen {
     Exclusive(MonitorHandle, VideoMode),