server: process fb update requests in fb update request message handler

Otherwise, the client won't get the initial frame

src/server.c

@@ -633,6 +633,8 @@ static int on_client_fb_update_request(struct nvnc_client* client)
 	if (fn)
 		fn(client, incremental, x, y, width, height);
 
+	process_fb_update_requests(client);
+
 	return sizeof(*msg);
 }