a1b0d2c933
The ViewerEndpoint can now send Snapshot messages to the SimEndpoint. The SimEndpoint has a new method, `get_viewer_snapshots`, to retrieve these. Thsi channel will be used for UI state needed for the simulation (e.g., `MjOption` and `StepControl`). PiperOrigin-RevId: 940981799 Change-Id: Ib9bd6ad848cd5479bc49492a34fddd82569cb09f