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-web/src/cursor.rs

@@ -54,6 +54,7 @@ impl CustomCursor {
                     true,
                 )
             },
+            _ => unimplemented!("unknown `CustomCursorSource` variant"),
         }
     }
 

winit-web/src/web_sys/event.rs

@@ -153,7 +153,7 @@ pub fn pointer_source(event: &PointerEvent, kind: PointerKind) -> PointerSource
 
             PointerSource::TabletTool { kind: tool, data }
         },
-        PointerKind::Unknown => PointerSource::Unknown,
+        _ => PointerSource::Unknown,
     }
 }
 

winit-web/src/web_sys/fullscreen.rs

@@ -88,6 +88,7 @@ pub(crate) fn request_fullscreen(
                 canvas.webkit_request_fullscreen();
             }
         },
+        _ => error!("This full screen mode is not supported"),
     }
 }
 

winit-web/src/web_sys/pointer.rs

@@ -90,7 +90,7 @@ impl PointerHandler {
                     PointerSource::TabletTool { kind, data } => {
                         ButtonSource::TabletTool { kind, button: event::tool_button(button), data }
                     },
-                    PointerSource::Unknown => ButtonSource::Unknown(button),
+                    _ => ButtonSource::Unknown(button),
                 };
 
                 handler(
@@ -152,7 +152,7 @@ impl PointerHandler {
 
                         ButtonSource::TabletTool { kind, button: event::tool_button(button), data }
                     },
-                    PointerSource::Unknown => ButtonSource::Unknown(button),
+                    _ => ButtonSource::Unknown(button),
                 };
 
                 handler(
@@ -229,7 +229,7 @@ impl PointerHandler {
                             button: event::tool_button(button),
                             data,
                         },
-                        PointerSource::Unknown => ButtonSource::Unknown(button),
+                        _ => ButtonSource::Unknown(button),
                     };
 
                     button_handler(