Option to disable sun texture

[llbit]

There should be an option to disable the sun texture when rendering.

[resist888]

This would be great to simulate night.

Also, if the sky is disabled - the sky could turn to stars (replacing the skymap unless the user overrides the option).

[llbit]

The commit that closed this issue only disables the sun texture when sunlight is disabled. There still needs to be an option to disable the sun texture.