Added a property for setting the amount of the vertex shadow attenuat…

[calogeromauceri]

Fixes #10913

Added an option to Globe (Globe.vertexShadowDarkness) for changing the amount of the terrain vertex shadow darkness when lighting is enabled (Globe.enableLighting). Similarly to ShadowMap.darkness parameter, a value near 0 makes the shadows darker, increasing it makes the shadows brighter. The default value is 0.3 (that was hard coded in Cesium).
Having the ability to change the terrain vertex shadow darkness allows to create more realistic shadows for cases, like the Moon, where there is no atmosphere and the light contribution from other sources (sun or earth) can be very small.

Here is a sandcastle example with default terrain vertex shadow darkness (current behaviour, advance the time in timeline till the terrain is fully shadowed)

sandcastle

here is a sandcastle example with the new parameter for changing the terrain vertex shadow darkness

sandcastle

The parameter can be dynamically changed, as visible in this example

sandcastle

[cesium-concierge]

Thanks for the pull request @calogeromauceri!

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Reviewers, don't forget to make sure that:

• Cesium Viewer works.
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[ggetz]

Thanks @calogeromauceri for the PR!

This is working well! I have a couple comments, then @sanjeetsuhag would you be able to take a pass on the globe shader changes?

[calogeromauceri]

@ggetz, @sanjeetsuhag thanks for your feedback. I just pushed the changes you suggested. Please let me know if anything else is needed

[sanjeetsuhag]

@calogeromauceri Using a uniform is working great. However, there's still some cleanup needed since the changes to GlobeSurfaceShaderSet are no longer needed.

[calogeromauceri]

@sanjeetsuhag I just pushed the changes you suggested. Please let me know if anything else is needed

[sanjeetsuhag]

Thanks for your patience with the review, @calogeromauceri. The changes look good! Thanks again!

[calogeromauceri]

@sanjeetsuhag thanks to you for the great support, guidelines and responsiveness!