Fix process hang on viewer close in launch_passive

WaitForAtomicNoGil spins until the render loop consumes a pending
set_texts/set_figures/set_images request (atomic == expected). If the
user closes the viewer window after a request is submitted but before
the render loop consumes it, the render loop exits and the atomic is
never updated. The calling thread spins forever, preventing the
process from terminating.

Check exitrequest inside the spin loop so the wait bails out once the
viewer is shutting down.

python/mujoco/simulate.cc

@@ -247,6 +247,11 @@ class SimulateWrapper {
     if (simulate_) {
       py::gil_scoped_release no_gil;
       while (atomic.load() != expected) {
+        // If the viewer is exiting, stop waiting. The render loop will
+        // never consume the pending request once it has set exitrequest.
+        if (simulate_->exitrequest.load()) {
+          return true;
+        }
         // TODO(robotics-simulation): replace with `atomic.wait(expected)` when
         // we migrate python bindings to C++20 (we may need to drop GCC 10).
         std::this_thread::yield();