fix: shadowMode cannot apply to i3dm tileset.

[JiaoJianing]

I tried to create a Cesium.Cesium3DTileset with the shadowMode: Cesium.ShadowMode.CAST_ONLY. The tileset contains many trees with i3dm format. But It looks like that shadowMode cannot apply to the tree Model. They will always receive shadow.
I finally find the reason: the shadowMode did not pass to the Model.

[cesium-concierge]

Thanks for the pull request @JiaoJianing!

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[OmarShehata]

Thanks @JiaoJianing - this seems like a pretty straightforward fix. Do you have a sample 3D Tileset you can share that we can use to test this?

Easiest way to share it would be to upload it to your Cesium ion account (https://cesium.com/ion/) and share a Sandcastle link here.

[JiaoJianing]

Hi~ @OmarShehata
I shared a Sandcastle link below, please check it!

https://sandcastle.cesium.com/#c=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

[JiaoJianing]

@OmarShehata
I also found that shadows state of i3dm cannot update dynamic. So I added the update logic in ModelInstanceCollection.prototype.update() to update the _model.shadows.

[lilleyse]

@JiaoJianing @OmarShehata it looks like this was actually just fixed in #8981. Back face culling, wireframe, and shadows all had the same problem.

This sandcastle should work in master:

http://localhost:8080/Apps/Sandcastle/index.html#c=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

[JiaoJianing]

@lilleyse @OmarShehata
OK! So I will close this PR.