fix Label backgroundPadding and backgroundColor

[nmschulte]

clone initial values to match setter behavior and other property behavior

[cesium-concierge]

Thanks for the pull request @nmschulte!

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• Works in 2D/CV.
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[hpinkos]

Good catch @nmschulte, thanks! I just made a few small changes so the function avoids creating an extra new Color and new Cartesian2 when those values were overridden by options passed in by the user.