perf(physics): vehicles standing still and held by their brakes let their physics sleep (physics.sleepheld)

application/driveruipanels.cpp

@@ -1513,6 +1513,8 @@ debug_panel::render_section_renderer() {
     }
     ImGui::PopTextWrapPos();
     ImGui::PopStyleColor();
+    // the physics of the vehicles, timed in the section of the main loop
+    ImGui::Checkbox( "let vehicles standing still and held by their brakes sleep (physics.sleepheld)", &Global.physics_sleepheld );
     return true;
 }