use single color to render the back side of terrain

Thanks for improving the underground navigation experience. It's important when dealing with some underground project. But sometimes it's hard to distinguish if the current view point is underground because the terrain is rendered the same on both sides. Maybe we can make it more clear by using single color to render the back side of terrain.

The issue names no file, test, or entry point. Start by locating Cesium's terrain rendering path and the handling of front- versus back-side terrain, then verify the underground view renders the back side with one distinct color while the normal view remains usable.

Written by the indexing model from the issue text.