Fixed 3D Tiles refinement bug when skipLevelOfDetail is false

[lilleyse]

Fixes #5814

This bug was introduced in #5788 which checks for a tile's visibility before loading it. This caused problems during the base-traversal step where it is expected that a replacement refinement tile loads all of its children, even if they are not visible. Otherwise the parent tile can never resolve.

This is not a problem with the internal-traversal (the skipping traversal), but since skipLevelOfDetail=false only uses the base-traversal it becomes a problem.

This PR limits the visibility check to additive tiles only. I think the visibility check would also work with replacement tiles during the skip traversal, but in practice this only saves a few requests and I don't want to mess with anything else.

[cesium-concierge]

@lilleyse, thanks for the pull request! Maintainers, we have a signed CLA from @lilleyse, so you can review this at any time.

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[bagnell]

Looks good to me.

@lilleyse Is there a reasonable test you can add?

[lilleyse]

Hmm I'll try to think of one.

[lilleyse]

Added a test, it checks the combined functionality of this PR, #5778, and #5788, which are all basically related.

[bagnell]

Thanks @lilleyse!