Preload terrain and imagery tiles

Hi,

I noticed in a recent release support was added for this:

`Added Cesium3DTileset.preloadFlightDestinations option to preload tiles at the camera's flight destination while the camera is in flight.`

However, that does not yet terrain and imagery tiles, correct? Would it be easy (or any technical pointers), to also implement this?

The issue points to Cesium3DTileset.preloadFlightDestinations as the existing related option, but names no files or tests. Start by locating that option and tracing how terrain and imagery tiles are requested during camera flight. Done means terrain and imagery at the flight destination are also preloaded without breaking existing behavior, with relevant tests updated or added.

Written by the indexing model from the issue text.