Implement scene switching mechanism

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Details:
The game currently only has one scene: the one where you're in a space ship.

Technically, it has two scenes; the world where the local star / local planets / far-away stars reside, and the space ship scene. These were separated as part #7 to make large-scale lighting easier. These two scenes still function as one however, and work together to facilitate space travel.

We need a mechanism to allow switching to / from scene groups in a clean way. This will eventually be useful for the galaxy map (or a potential main menu), but has an immediate use-case: testing star catalogs and shaders via free-flying through stars.

(The honest truth is that flying through real stars is really addictive and I just want it kept as a separate game mode.)

Task:

• Implement a mechanism that allows switching between scene groups.
• Add the option to switch the star free-flight in the debug menu (which should eventually probably be moved to an astrometrics menu).

This should support:

• Changing renderers (or changing renderer options). The reason for this is that stars require disabling depth testing, which messes with space ship rendering, so options need to be adjustable during the change.
• Changing shaders.
• Changing space ships.
• Understanding which scenes belong together and which don't.
• We should have a manifest (or module?) that describes switching functionality. For example, when switching to space ship, we do not want the space ship sent back to 0,0,0. Or, maybe we even want the space scene to keep running, just without having control over rendering. Perhaps separate space ship logic from scene logic entirely so that completely killing the scene and resetting to default during the switch does not impact gameplay.

Some considerations:
Space ships inhabited by the controlling player are seen as levels from the game's point of view. It's possible we'll need some level logic running in the background. However, slow or dangerous logic such as "calculate distance to door to see if it can be opened" would potentially need to be stopped if switching to another scene. Decide if it's within scope during implementation.

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Scene switching can be tested by going to Menu -> Debug Tools -> Star free-flight.

This will switch to a scene where you can freely fly through near-Earth stars. In the background, space ship processing will continue even though the ship and its surrounding are no longer being rendered.