Fix IPC socket deleted on multi-output setups

[apostolosSotiropoulos]

The i3-ipc plugin extends per_output_plugin_instance_t, so one instance is created per output. This caused two issues:

1. serve() was called once per output, each call unlinking the previous socket file before creating a new one. With N outputs, the socket was created and deleted N times.

2. When an output was disabled/removed, fini() triggered handle_display_destroy() which closed and unlinked the socket, even though other output instances still needed it.

Fix serve() to only initialize the socket once (guard on ipc_socket > 0), and simplify fini() to only unbind events without destroying the server. The server cleanup on actual shutdown is already handled by the wl_display_add_destroy_listener registered in serve().

[AR-CADE]

I think you're right, this plugin shouldn't be instantiated "per output" because it can lead to unexpected behaviors, and what's more, it's useless in this case...

this plugin was initially a singleton plugin, so replacing "per_output_plugin_instance_t" with "plugin_interface_t" should do the trick.

My advice is to revert your changes and try to do only this:

-#include "wayfire/per-output-plugin.hpp"
+#include <wayfire/output-layout.hpp>
+#include <wayfire/plugin.hpp>


-class ipc_t : public wf::per_output_plugin_instance_t
+class ipc_t : public wf::plugin_interface_t


-DECLARE_WAYFIRE_PLUGIN(wf::per_output_plugin_t<ipc_t>);
+DECLARE_WAYFIRE_PLUGIN(ipc_t);

Let me know if it works for you, and if so, can you please modify your PR to only contains thoses changes,

thanks...