Fixed bug when switching to orthographic camera on first frame

[IanLilleyT]

Don't merge until #8850 is ready.

When going from perspective to orthographic, it does a depth pick to figure out what size to make the frustum. The closer you are to an object, the smaller the orthographic frustum.

This was failing on the first frame because it was reading a garbage depth value, since nothing had been renderer yet. This PR adds a check for this and uses the cartographic height as a fallback.

There's another fix in here which makes sure to reconstruct the world space position using the correct frustum. The old code was reconstructing the perspective-generated depth with the new orthographic frustum, resulting in an incorrect world position. Now it reconstructs with the perspective frustum before changing to orthographic.

Before:

After:

[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]

Confirmed that this fixes the problem of the orthographic frustum not being sized correctly on the first frame.

Local sandcastle vs. hosted sandcastle