Issue with differing imagery zoom levels

We are experiencing an issue similar to the one described [here](https://groups.google.com/forum/#!topic/cesium-dev/QGP3mwnPawA/discussion).

Essentially, the zoom level for any given tile on the globe is determined by its distance from the camera. For a "roughly top-down" view (however we decide to define that), we would prefer all visible tiles to be the same zoom level. This helps tiles that contain text, heatmaps, or other important information to line up properly with the adjacent tiles.

I know that the solution in that groups discussion is both hacky and very old. I poked around and the `screenSpaceError()` function appears to have moved to a closure within `Scene/QuadtreePrimitive.js`. Is it possible to expose it on the API in some way that would allow it to be overwritten or replaced?

If not, is there some other way of achieving a more uniform level of detail for this imagery?

Start with the screenSpaceError() closure in Scene/QuadtreePrimitive.js and review the linked discussion for the existing approach. Determine whether exposing or replacing that logic can enforce a uniform imagery level of detail in roughly top-down views, and define a supported API or alternative that keeps visible adjacent tiles at the same zoom level.

Written by the indexing model from the issue text.