Fix issue where unpausing the simulation (especially under slowdown) causes a jump in simulation time, in both native simulate and Python viewer.

PiperOrigin-RevId: 568948943
Change-Id: I2f401508555e272f348e97627aeb85cb3e9cf017

python/mujoco/viewer.py

@@ -298,6 +298,7 @@ def _physics_loop(simulate: _Simulate, loader: Optional[_InternalLoaderType]):
 
           # Run mj_forward, to update rendering and joint sliders.
           mujoco.mj_forward(m, d)
+          synccpu = time.time()
 
 
 def _launch_internal(

simulate/main.cc

@@ -392,6 +392,7 @@ void PhysicsLoop(mj::Simulate& sim) {
         else {
           // run mj_forward, to update rendering and joint sliders
           mj_forward(m, d);
+          syncCPU = mj::Simulate::Clock::now();
         }
       }
     }  // release std::lock_guard<std::mutex>