Back face culling override for 3D Tiles and models

[lilleyse]

Many 3D Tilesets and glTF models set the double-sided property to false but CesiumJS doesn't provide a way to override this behavior in case you want to view the model from below or inside, and don't have control over the content generation pipeline to fix the glTF directly.

globe.backFaceCulling was recently added, we should consider adding the same to tilesets and models.