Improve underground rendering

[lilleyse]

Detects if the camera is underground, and if it is:

• Disable skirts
• Disable back-face culling
• Disable fog
• Disable ellipsoid occlusion culling for both terrain tiles and primitives. Primitives inside the globe will not be culled.

Known issues that don't need to hold up this PR:

• Because skirts are disabled it's easy to see through terrain seams. Instead it would better to invert the skirts.
• The depth plane is non-existent when you're underground so stuff above the surface on the opposite side of the globe will be visible. Like in this sandcastle (click "Add points")

Don't merge until after the February release, this needs more time to sit in master. Camera navigation PR to follow this one.

@fredj @gberaudo

[cesium-concierge]

Thanks for the pull request @lilleyse!

• ✔️ Signed CLA found.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[hpinkos]

@IanLilleyT @lilleyse is this still waiting for a review?

[lilleyse]

@IanLilleyT's going to review after Friday, it's ok if this doesn't make the March release.

[IanLilleyT]

Looks good to me, thanks @lilleyse !