Kill the server only after some delay

Let the server terminate properly once all the sockets are closed.

If it does not terminate (this can happen if the device is asleep), then
kill it.

Fixes #1992 <https://github.com/Genymobile/scrcpy/issues/1992>

app/src/server.h

@@ -18,6 +18,11 @@ struct server {
     process_t process;
     SDL_Thread *wait_server_thread;
     atomic_flag server_socket_closed;
+
+    SDL_mutex *mutex;
+    SDL_cond *process_terminated_cond;
+    bool process_terminated;
+
     socket_t server_socket; // only used if !tunnel_forward
     socket_t video_socket;
     socket_t control_socket;