Otherwise it's theoretically possible to create a new ClientConnection
object for the zombie wl_client when its resources are being destroyed.
For example
- process early wl_client destroy notification
- the ClientConnection objects gets removed from the client list in Display
- process wl_resource objects getting destroyed
- if some code calls display->getConnection(zombie_client), it's going to
reintroduce the client in the client list
- process late wl_client destroy notification, it's going to destroy the
original and the clone ClientConnection object
This change prevents reintoducing a clone client object, by keeping the
original for a bit longer until it's actually destroyed. In the future though,
it would be great to kill the client lists in Display and ClientConnection,
and just use `static_cast<ClientConnection *>(wl_client_get_user_data())`.
