Add support for user-defined figures in the MuJoCo viewer.

PiperOrigin-RevId: 726983516
Change-Id: Ifb821f3cafb163d4f3077d76b0668037b4bc5a9c

simulate/simulate.cc

@@ -542,6 +542,10 @@ void ShowSensor(mj::Simulate* sim, mjrRect rect) {
   mjr_figure(viewport, &sim->figsensor, &sim->platform_ui->mjr_context());
 }
 
+void ShowFigure(mj::Simulate* sim, mjrRect viewport, mjvFigure* fig){
+  mjr_figure(viewport, fig, &sim->platform_ui->mjr_context());
+}
+
 // load state from history buffer
 static void LoadScrubState(mj::Simulate* sim) {
   // get index into circular buffer
@@ -2588,6 +2592,11 @@ void Simulate::Render() {
     }
   }
 
+  // user figures
+  for (auto& [viewport, figure] : this->user_figures_) {
+    ShowFigure(this, viewport, &figure);
+  }
+
   // finalize
   this->platform_ui->SwapBuffers();
 }