Objectron broken w/ new blueprint model

[jleibs]

Easy repro by just running objectron.

Appears related to the rendering of the world/camera entity as disabling it makes the behavior go away.

It seems like we might be toggling between treating the scene as a 2d vs a 3d view.

The space-view flickers between two states:

[jleibs]

It looks like EditableAutoValue is the cause of the instability here.
Somehow the result of:

self.nav_mode = EditableAutoValue::Auto(scene.preferred_navigation_mode(space));

after through through a serialize/deserialize cycle is unstable.