Support a depthFailMaterial for 3D Tiles

The [1.70 release introduced improvements for underground visualization](https://cesium.com/blog/2020/06/01/cesium-june-release/). One thing that would help a lot is the ability to see what parts of a 3D Tileset are underground, similar to how you can create a depth fail material for polylines. This has a few advantages over just use globe translucency:

* Shows the underground data fully without needing to define a cutout rectangle or figure out a near/far distance for transparency
* Makes it easier to see exactly where the cut-off level between above ground/under ground is

Start by reviewing the existing depth-fail material behavior for polylines and the underground visualization improvements described in the linked 1.70 release post. Define the design and validation needed for 3D Tilesets to show underground portions with a depth-fail material, including how the above-ground/underground cutoff should appear.

Written by the indexing model from the issue text.