Fix IE crash

[lilleyse]

#8684 broke ground atmosphere rendering in IE. After some investigation it looks like the cause is a faulty GLSL implementation.

This old commit helped me out: 28d15df.

Needs to be merged before April 1.

[cesium-concierge]

Thanks for the pull request @lilleyse!

• ✔️ 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.

[OmarShehata]

I can't seem to reproduce a crash in IE11 in master. This is my IE version:

I am comfortable merging this pre-computed version though given this was something we had intentionally fixed the same way when we ran into before.

The failing test on Travis is succeeding locally. I can't seem to restart the Travis build myself though.

[lilleyse]

The failing test on Travis is succeeding locally. I can't seem to restart the Travis build myself though.

I can't restart it either...