Fixed horizon culling issues with large root tiles

[IanLilleyT]

There were some horizon culling bugs with ArcGisTerrainProvider's root tile which cause some child tiles to be culled or the whole screen to go black altogether. The problem is the tile was creating a horizon culling point from a set of points that were on opposite sides of the globe. This has been a bug for a while but was made worse by #8475. The fix was to detect these backwards points in EllipsoidalOccluder and throw out the entire occlusion point if one of them is backwards. In practice this only happens for very large tiles with widths greater than half the globe. CesiumWorldTerrain never had this problem because it has two root tiles. ArcGisTerrainProvider only has one root tile so it was affected.

Before sandcastle

After sandcastle (local)

[cesium-concierge]

Thanks for the pull request @IanLilleyT!

• ✔️ Signed CLA found.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

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

[lilleyse]

Thanks, this fixes the problem we were seeing last night.

I confirmed again that this doesn't change the number of tiles rendered each frame. Sandcastle with all the different terrain providers

[IanLilleyT]

@lilleyse I added some new test cases. Ready for review.

[lilleyse]

Thanks @IanLilleyT