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

doc/changelog.rst

@@ -48,9 +48,12 @@ General
   the free camera at model load time.
 - Added ``mjv_defaultFreeCamera`` which sets the default free camera, respecting the above attributes.
 - ``simulate`` now supports taking a screenshot via a button in the File section or via ``Ctrl-P``.
+- Improvements to time synchronisation in `simulate`, in particular report actual real-time factor if different from
+  requested factor.
 - Added a disable flag for sensors.
 - :ref:`mju_mulQuat` and :ref:`mju_mulQuatAxis` support in place computation. For example
   |br| ``mju_mulQuat(a, a, b);`` sets the quaternion ``a`` equal to the product of ``a`` and ``b``.
+
 Deleted/deprecated features
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^