Added CubeTexture().fromEquirectangular()

[spite]

Method and examples

[armurei]

What's the status of this PR? As far as I know there is still no easy way to set Scene.background to an equirectangular texture, so this would be very helpful.

[Mugen87]

From my point of view, using CubeTexture.fromEquirectangular() is actually a nicer workflow than working with a separate class. Maybe there is a way to move the existing code of EquirectangularToCubeGenerator into the core?

[WestLangley]

@spite I think this is valuable. Can you please resolve the conflicts so we can try again?

[Mugen87]

Closing in favor of #16671