Tootsville.SkyBuilder.build is a function with lambda list: (world)
Build the sky for the current environment. Reads the sky values at
Tootsville.SkyBuilder.sky and affects the scene at Tootsville.Tank.scene.
Depending on the world in question, the sky may have these layers:
The first layer is the base color of the sky. On Chœrogyllum, this
varies from a blue bordering on white at noon (09:00) to pitch black
at midnight (00:00) and should have redness applied at sunset.
The ambient color lighting of the scene is likewise affected.
In space, it is always black.
The second layer is a starfield, which fades with the sun's altitude.
The third layer has the sun.
The fourth layer has The Moon, The Other Moon, and the Pink Moon, as
appropriate, and Chœrogryllum, when in space.
The fifth layer is cloud cover (when on the planet).
Finally, the sixth layer is any precipitation effect.