Fix running server and GUI together on macOS (#3405)

On macOS the Metal API requires initialization from the main thread. As a result, we currently cannot run worlds with rendering sensors on macOS without running the server and GUI seprately. This PR fixes this be ensuring the server runs on the main thread (using the blocking=true flag on Server::Run) and creating a separate thread to monitor the GUI process.

Generated-By: Gemini 2.5 Pro

Signed-off-by: Addisu Z. Taddese <addisuzt@intrinsic.ai>
Co-authored-by: Arjo Chakravarty <arjoc@intrinsic.ai>
(cherry picked from commit 7dd25f6)

Author: azeey

src/cmd/sim_main.cc

@@ -483,11 +483,8 @@ int main(int argc, char** argv)
         return -1;
     }
 
-    bool blocking = true;
-
     #ifdef WITH_GUI
     opt->waitGui = 1;
-    blocking = false;
 
     // Launch the GUI in a separate thread
     std::thread guiThread(
@@ -525,20 +522,27 @@ int main(int argc, char** argv)
       return -1;
     }
 
-    // Run the server in a separate thread
     sim::Server server(serverConfig);
-    server.Run(blocking, opt->iterations, opt->runOnStart == 0);
 
     #ifdef WITH_GUI
-    // Join the GUI thread to wait for a possible window close
-    if (guiThread.joinable())
-      guiThread.join();
+    // Watch for the GUI to close, and stop the server if it does.
+    std::thread watcherThread([&]{
+      // Wait for the GUI thread to finish, indicating the GUI has closed
+      if (guiThread.joinable())
+        guiThread.join();
 
-    // Shutdown server if the GUI has been closed from the screen
-    if(server.Running())
-    {
-      server.Stop();
-    }
+      if(server.Running())
+      {
+        server.Stop();
+      }
+    });
+    #endif
+
+    // Run the server on the main thread. This is a blocking call.
+    server.Run(true, opt->iterations, opt->runOnStart == 0);
+
+    #ifdef WITH_GUI
+    watcherThread.join();
     #endif
 
     gzdbg << "Shutting down gz-sim-server" << std::endl;