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/windows/event_loop.rs

@@ -1307,7 +1307,7 @@ unsafe extern "system" fn public_window_callback<T: 'static>(
                 event: MouseInput {
                     device_id: DEVICE_ID,
                     state: Pressed,
-                    button: Other(xbutton as u8),
+                    button: Other(xbutton),
                     modifiers: event::get_key_mods(),
                 },
             });
@@ -1329,7 +1329,7 @@ unsafe extern "system" fn public_window_callback<T: 'static>(
                 event: MouseInput {
                     device_id: DEVICE_ID,
                     state: Released,
-                    button: Other(xbutton as u8),
+                    button: Other(xbutton),
                     modifiers: event::get_key_mods(),
                 },
             });