wayland-server: Add wl_client_add_event_dispatch_listener

The patch adds a listener called after an event has been dispatched by the server.

Wayland servers that incorporate the Wayland display into their own event loops may dispatch multiple Wayland events to clients as they work through their task queues.

For e.g. if system display state changes the Wayland server may emit several output / surface Wayland events to connected clients.

Currently an explicit flush is performed immediately after each individual Wayland event is dispatched to make sure these are propagated to clients in a deterministic / reliable manner.

This results in hard to maintain / easy to break code. Contributors must make sure to explicitly flush when dispatching Wayland events to clients - or otherwise wait for some other server event to flush the client buffer later on. In the latter case behavior is not deterministic and makes testing the server code much harder.

The client event_dispatch_listener added in this patch allows the server to track if the processing of a server event has resulted in the dispatch of one or more Wayland events to clients in a single location - and ensure the buffer is appropriately flushed.

This eliminates the need to flush in many different areas of the codebase, making the server easier to maintain and test.

Signed-off-by: Thomas Lukaszewicz thomaslukaszewicz@gmail.com