Fix a few issues with the passive viewer.

1. Create an arena for the mjData instance used by the passive viewer visualization.

When using the passive viewer, stuff gets copied from the real mjData into a minimal struct.
That struct didn't have a stack, and now visualization for Flex does stack allocs.

2. Add missing a missing field in scene state for flex visualization.

3. Fix a memory leak where mjvScene wasn't released on exit in the passive viewer.

4. Add some locks in places where the render thread and Simulate::Sync collide.

This fixes #1280.

PiperOrigin-RevId: 591891676
Change-Id: I592f286cab9719c8d9af84e42f9756ea1f9f1971

include/mujoco/mjvisualize.h

@@ -436,6 +436,7 @@ struct mjvSceneState_ {
     int nnames;
     int npaths;
     int nsensordata;
+    int narena;
 
     mjOption opt;
     mjVisual vis;
@@ -649,6 +650,7 @@ struct mjvSceneState_ {
     mjtNum* ten_length;
     mjtNum* wrap_xpos;
 
+    mjtNum* bvh_aabb_dyn;
     mjtByte* bvh_active;
     int* island_dofadr;
     int* island_dofind;
@@ -660,6 +662,7 @@ struct mjvSceneState_ {
 
     mjContact* contact;
     mjtNum* efc_force;
+    void* arena;
   } data;
 };
 typedef struct mjvSceneState_ mjvSceneState;

include/mujoco/mjxmacro.h

@@ -614,7 +614,7 @@
     X   ( mjtNum,    qLD,               nM,          1           ) \
     X   ( mjtNum,    qLDiagInv,         nv,          1           ) \
     X   ( mjtNum,    qLDiagSqrtInv,     nv,          1           ) \
-    X   ( mjtNum,    bvh_aabb_dyn,      nbvhdynamic, 6           ) \
+    XMJV( mjtNum,    bvh_aabb_dyn,      nbvhdynamic, 6           ) \
     XMJV( mjtByte,   bvh_active,        nbvh,        1           ) \
     X   ( mjtNum,    flexedge_velocity, nflexedge,   1           ) \
     X   ( mjtNum,    ten_velocity,      ntendon,     1           ) \