On Wayland, don't drop extra mouse buttons

This commit forwards "unknown" Wayland mouse buttons downstream via
'MouseButton::Other'. Possible values for those could be found in
<linux/input-event-codes.h>.

Also, since Wayland just forwards buttons from the kernel, which are
'u16', we must adjust 'MouseButton::Other' to take 'u16' instead of
'u8'.

src/platform_impl/linux/x11/event_processor.rs

@@ -709,7 +709,7 @@ impl<T: 'static> EventProcessor<T> {
                                 event: MouseInput {
                                     device_id,
                                     state,
-                                    button: Other(x as u8),
+                                    button: Other(x as u16),
                                     modifiers,
                                 },
                             }),