Fix type-related issues in dependent code. Remove hardcoded mjUSEDOUBLE. Add mjUSESINGLE compiler flag.

This CL does not change the default build behavior of MuJoCo. To use single-precision floating-point, build MuJoCo with `-DmjUSESINGLE`.

PiperOrigin-RevId: 644782648
Change-Id: Ie815df9916798ca8054306437b39a33f84ce9e08

simulate/simulate.cc

@@ -1165,7 +1165,9 @@ void CopyPose(mj::Simulate* sim, const mjModel* m, const mjData* d) {
 
 // millisecond timer, for MuJoCo built-in profiler
 mjtNum Timer() {
-  return Milliseconds(mj::Simulate::Clock::now().time_since_epoch()).count();
+  static auto start = mj::Simulate::Clock::now();
+  auto elapsed = Milliseconds(mj::Simulate::Clock::now() - start);
+  return elapsed.count();
 }
 
 // clear all times