The "Listener" connection, being private and under the control of the
qemu display, allows for the optimization of discarding pending
intermediary messages when queuing a new scanout. This ensures that the
client receives only the latest scanout update, improving communication
efficiency.

While the current implementation does not provide a mechanism for
clients who may wish to receive all updates, making this behavior
optional could be considered in the future. For now, adopting this new
default behavior accelerates the communication process without a
guarantee of delivering all updates.

The filter is removed when the connection is dropped.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>