Don't overwrite scratch variables in entity visualizers

[hpinkos]

Fixes #6813

The example provided in #6813 was using a CallbackProperty for position and not using the result parameter. We were then overwriting the class scoped position scratch variable with the position returned from the CallbackProperty. BillboardVisualizer, LabelVisualizer and PointVisualizer all had this problem. None of the other visualizers did.

I changed the position variable to be in the function scope, and renamed all of the scratch variables to xxxScratch to avoid this kind of problem in the future.

I had no idea how to add a unit tests for this, but I think this kind of fix is unlikely to break again so I think it's okay without a new unit test.

[cesium-concierge]

Thanks for the pull request @hpinkos!

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[mramato]

Thanks @hpinkos!