If a user leave and then rejoins a multiplayer match while another user
is not at the lobby screen, there is a potential the `ParticipantPanel`
tracking the user will not correctly be recreated to reference the new
instance of the `MultiplayerUser`.
This happens because the `OnRoomUpdated` call is scheduled, which means
it is not running in the background, coupled with the local logic that
relies on `IEquatable(MultiplayerRoomUser)` (which in turn falls back to
a UserID comparison).
Changing this to a reference comparison is the easiest way to resolve
this. Whether we change the `IEquatable` implementation is up for
discussion.
Closes https://github.com/ppy/osu/issues/15970.
