Fix ground polyline with dynamic color

[hpinkos]

Fixes #6925

This is the same thing we do for other static geometry types, we must have just missed putting it in for ground polyline

[cesium-concierge]

Thanks for the pull request @hpinkos!

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[bagnell]

Code looks good. Can you add a test?

[hpinkos]

Thanks @bagnell, this is ready

[bagnell]

The test you added passes in master.

[hpinkos]

@bagnell fixed

[bagnell]

Thanks 👍