Improvements to time synchronisation in simulate.

- Improved time-sync logic readability in `PhysicsLoop`.
- Fixed unattainable condition for breaking from stepping loop if data was reset.
- Added measurement of actual real-time tracking.
- Moved slowdown overlay to top left.
- Report slowdown as % of real-time, rather than fraction.
- Report actual slowdown if different than requested (very small timestep, PhysicsLoop cannot keep up) by more than 10%.
- Replaced slowdown increments of 2 with decimal-rounded increments of 10^(1/10). This is a finer-grained scale (approximately 3x as fine) and well suited for decimal percentage representation.
- Maximum slowdown increased to 1000.
- Added `Simulate.refreshrate` with a default of 60Hz, in case videomode refreshrate is 0 (can occur e.g. when forwarding over X11).
- Remove realtime reporting from regular info overlay.
- Various cleanups.

PiperOrigin-RevId: 471547481
Change-Id: Idd2514a4defb1dc431617f3a515cc747c4fd8971

simulate/simulate.h

@@ -86,9 +86,7 @@ class MJSIMULATEAPI Simulate {
   std::mutex mtx;
   std::condition_variable cond_loadrequest;
 
-  std::atomic_bool exitrequest = false;
-
-  // option
+  // options
   int spacing = 0;
   int color = 0;
   int font = 0;
@@ -102,24 +100,43 @@ class MJSIMULATEAPI Simulate {
   int vsync = 1;
   int busywait = 0;
 
-  // simulation
-  int run = 1;
   // keyframe index
   int key = 0;
-  std::atomic_int uiloadrequest = 0;
+
+  // simulation
+  int run = 1;
+
+  // atomics for cross-thread messages
+  std::atomic_bool exitrequest = false;
   std::atomic_bool droploadrequest = false;
-  // 2: render thread asked to update its model
-  // 1: showing "loading" label, about to load
-  // 0: model loaded or no load requested.
-  int loadrequest = 0;
   std::atomic_bool screenshotrequest = false;
+  std::atomic_int uiloadrequest = 0;
+
+  // loadrequest
+  //   2: render thread asked to update its model
+  //   1: showing "loading" label, about to load
+  //   0: model loaded or no load requested.
+  int loadrequest = 0;
+
   // strings
   char loadError[kMaxFilenameLength] = "";
   char dropfilename[kMaxFilenameLength] = "";
   char filename[kMaxFilenameLength] = "";
   char previous_filename[kMaxFilenameLength] = "";
-  int slow_down = 1;
-  bool speed_changed = true;
+
+  // time synchronization
+  int realTimeIndex = 0;
+  bool speedChanged = true;
+  float measuredSlowdown = 1.0;
+  // logarithmically spaced realtime slow-down coefficients (percent)
+  static constexpr float percentRealTime[] = {
+      100, 80, 66,  50,  40, 33,  25,  20, 16,  13,
+      10,  8,  6.6, 5.0, 4,  3.3, 2.5, 2,  1.6, 1.3,
+      1,  .8, .66, .5,  .4, .33, .25, .2, .16, .13,
+     .1
+  };
+
+  // control noise
   double ctrlnoisestd = 0.0;
   double ctrlnoiserate = 0.0;
 
@@ -147,6 +164,7 @@ class MJSIMULATEAPI Simulate {
 
   // OpenGL rendering and UI
   GLFWvidmode vmode = {};
+  int refreshRate = 60;
   int windowpos[2] = {0};
   int windowsize[2] = {0};
   mjrContext con = {};