7cb7c87f70
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