Fixed depth plane for orthographic

[IanLilleyT]

This PR fixes the polyline subissue in #5110 (which is not actually related to ground primitives).

Two fixes went into this

1. Keep the orthographic depth plane centered at the ellipsoid center instead of moving closer to the camera.
2. In the depth plane shader when intersecting the ellipsoid, need to shoot rays from the near plane all in the same direction.

Before Sandcastle (look at the left edge of the globe)

After Sandcastle

[cesium-concierge]

Thanks for the pull request @IanLilleyT!

• ✔️ 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.

[he-hesce]

@lilleyse: Would like to see this merged and included in 1.70.

Thanks to @IanLilleyT for addressing this critical issue #5110 for us. 👍 🥇

[mramato]

Does this fix #8653?

[IanLilleyT]

@mramato: Nope, that bug is unrelated to depth plane, I added more info there: #8653.

[lilleyse]

Looks good for this sandcastle too: sandcastle

master

orthoDepthPlaneFix

[lilleyse]

Code looks good 👍

[he-hesce]

Thanks @lilleyse! 💯