ShadowMap fade option

[kalosma]

Fixes #9564

Added an option to shadowMap (shadowMap.fadingEnabled) for enabling/disabling the shadowMap fading when the light source is close to the horizon.
Having the fading disabled is desirable for cases, like the Moon, where there is no atmosphere and the shadows should be solid even if the light source is close to the horizon.

here is a sandcastle example with fading enabled (current behaviour, advance the time in timeline till the shadows fade)

fading enabled (default)

here is a sandcastle example with the new option to disable the shadows fading

fading disabled (through new option)

[cesium-concierge]

Thanks for the pull request @kalosma!

• ✔️ 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.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[ebogo1]

Thanks @kalosma! I confirmed these changes work in Sandcastle. A few things -

[kalosma]

Had some problems with the unit test, but it seems it passed now

[ebogo1]

Thanks, this PR looks just about ready. Just a minor change in wording for the test case and a comment I left earlier that you might've missed (I unresolved it). Should be good to go after those changes.

[ebogo1]

Thanks @kalosma. I pushed a couple changes to CHANGES.md but this is good to go 👍