Night textures don't work with models

[SevenSpheres]

Describe the bug Night textures don't display on objects with models.

To Reproduce Use some code like this:

Modify "Vesta" "Sol" {
	NightTexture "earth-night.*"
}

Expected behavior The night side texture is displayed.

Desktop (please complete the following information):

• OS: Windows 10, macOS Catalina
• Frontend: native Win, native Mac
• Version: 1.6.2 (beta3)

Additional context Reported by The Minmus Derp, confirmed by me

[375gnu]

@SevenSpheres could you provide a reasonable usage for this feature?

[SevenSpheres]

In Minmus' case, he was making a (fictional) hot asteroid with a glowing night side texture.

[375gnu]

This is job for Emissive texture.