Alpha blending for imagery layers LODs

I _think_ this would be rather straightforward - it came up [on the forum](https://community.cesium.com/t/cesium-and-mapbox-how-to-have-the-same-nice-feeling/8619) here that it would be nice to have imagery layers fade in/out instead of popping in when loading levels of detail. It should definitely be an option you can turn off since it adds a little bit of a delay for when the LOD appears.

The issue points to imagery-layer LOD loading and the linked forum discussion; start by tracing how imagery LOD changes are currently displayed. Check the existing behavior and determine how an opt-out should work, then verify that enabled fading prevents visible popping without making LOD visibility unacceptably delayed.

Written by the indexing model from the issue text.