Expose a handle for the Python viewer.

This change also requires user scripts to explicitly synchronize changes to physics state to the viewer. The Simulate class was reconfigured so that certain UI events are handled during this sync operation, outside of the render loop on the main thread. These correspond to operations that require access to the full mjModel/mjData.

To support other, more interactive operations (e.g. camera movements), a new mjvSceneState struct is introduced which captures only the portion of the physics state required for scene re-rendering. The mjvSceneState is updated from mjModel/mjData during the viewer sync operation, and is significantly cheaper than a full mj_copyModel and mj_copyData.

Fixes https://github.com/deepmind/mujoco/issues/796

PiperOrigin-RevId: 525723636
Change-Id: Id08d0210a2c067d5afe85e2bf104f276aeddd75e

python/mujoco/render.cc

@@ -250,6 +250,7 @@ PYBIND11_MODULE(_render, pymodule) {
   Def<traits::mjr_changeFont>(pymodule);
   Def<traits::mjr_addAux>(pymodule);
   // Skipped: mjr_freeContext (have MjrContext.__del__)
+  Def<traits::mjr_resizeOffscreen>(pymodule);
   Def<traits::mjr_uploadTexture>(pymodule);
   Def<traits::mjr_uploadMesh>(pymodule);
   Def<traits::mjr_uploadHField>(pymodule);