Fix for walls with negative heights

[hpinkos]

Fixes #4274
Replaces #9022

sandcastle

The removeDuplicates function in WallGeometryLibrary was making the incorrect assumption that the minimum height of the bottom of the wall would be 0, and if all positions at the top of the wall were <=0 then the height of the wall was 0 and we didn't need to render anything. This updates the removeDuplicates function to check if topHeight === bottomHeight instead of topHeight <= 0 to determine if the wall positions resulted in a wall with 0 height.

Also fixed the logic for computing wall normals so it works when the top of the wall was at height === 0 by using a position that is 1m below the top wall position instead of a position with height === 0 for the cross product computation.

[cesium-concierge]

Thanks for the pull request @hpinkos!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ 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.

[hpinkos]

@lilleyse can you review, or recommend someone to review? Thanks!

[lilleyse]

I see @YoussefV was tagged in #9044.

@YoussefV are you good with reviewing this one as well afterwards?

[YVin3D]

For future people looking over this: here is another example of a sandcastle that shows the issue (adapted from #4274) You should be seeing a red wall. Previous code did not show a red wall