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/cursor.rs

@@ -111,6 +111,7 @@ impl_dyn_casting!(CustomCursorProvider);
 ///
 /// See [`CustomCursor`] for more details.
 #[derive(Debug, Clone, Eq, Hash, PartialEq)]
+#[non_exhaustive]
 pub enum CustomCursorSource {
     /// Cursor that is backed by RGBA image.
     ///
@@ -167,6 +168,7 @@ impl CustomCursorSource {
 /// An error produced when using [`CustomCursorSource::from_rgba`] with invalid arguments.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[non_exhaustive]
 pub enum BadImage {
     /// Produced when the image dimensions are larger than [`MAX_CURSOR_SIZE`]. This doesn't
     /// guarantee that the cursor will work, but should avoid many platform and device specific
@@ -217,6 +219,7 @@ impl Error for BadImage {}
 /// An error produced when using [`CustomCursorSource::from_animation`] with invalid arguments.
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[non_exhaustive]
 pub enum BadAnimation {
     /// Produced when no cursors were supplied.
     Empty,