Fog post process sandcastle

[lilleyse]

While working on globe translucency I created a fog post process effect that might be nice to include in sandcastle. It's implemented with a custom post process where the fog distance and color are uniforms and can be changed dynamically.

[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.

[cesium-concierge]

Thanks again for your contribution @lilleyse!

No one has commented on this pull request in 30 days. Maintainers, can you review, merge or close to keep things tidy?

I'm going to re-bump this in 30 days. If you'd like me to stop, just comment with @cesium-concierge stop. If you want me to start again, just delete the comment.

[hpinkos]

Thanks @lilleyse! This is a great demo =)

[hpinkos]

Added a comment to CHANGES.md here 7efc915