Better lighting for asset preview

[mramato]

Currently, the asset preview window uses the browser's current local time, so if the asset being previewed is in a different part of the world (or it's night on the client), the asset is completely unlit and the preview is useless.

In the short term, we can fix this by computing when the sun will be overhead the center of the asset and use that time instead.

In the longer term, I wrote up CesiumGS/cesium#6553 so that CesiumJS provides us more lighting options.

[mramato]

Shipped as of yesterday.