A few small fixes from local testing

[ryantrem]

I checked out the branch locally to test and found a few small issues, which this PR fixes:

1. We need to store _skyboxVisible separate from the skybox mesh, since the environment visible flag could be set before a skybox has been loaded (or when switching to a new environment, in which case we should preserve the visibility flag).
2. Update how we handle the Scene.autoClear flag, such that autoClear is enabled if there is no skybox or if the skybox is not visible.
3. Wrapped some of the new state mutations in calls to disableSnapshotRendering and enableSnapshotRendering to ensure that the snapshot rendering state is correctly updated (needed for WebGPU).