Add "backFaceCulling" option to Globe

[OmarShehata]

This came up on Stackoverflow:

https://stackoverflow.com/questions/53277048/how-do-i-clip-an-entity-or-geometry-against-underside-of-terrain

It seems to be a common scenario to isolate a piece of terrain like that (common enough that we have a Sandcastle for it). But one issue is that since backfaces are culled, you can see through the terrain, and that looks ugly.

If you disable backface culling, it looks great:

I did this by manually editing the source here:

https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Scene/GlobeSurfaceTileProvider.js#L436-L458

It would be nice if this was an option. I think it could be part of the effort to give users more control over blend mode settings #7203. This particular case could easily be just a boolean you can set on the globe.

[IanLilleyT]

Added backFaceCulling option to Globe in #8470.

globe.backFaceCulling = false fixes this issue.