Terrain upsampling

- Both the terrain data from the server and the uncompressed or packed mesh is kept in memory. The data from the server is kept for upsampling. The uncompressed or packed data is kept for picking. Refactor upsampling to use the uncompressed/packed mesh.
- Do we need parallel upsampling? Investigate only using upsampling at the leaf nodes. Currently, terrain is upsampled at the same time a new tile is requested with the idea that, if the imagery is loaded faster than terrain, it will be displayed as soon as possible. If the terrain loads faster than the upsampling, the upsampled terrain is thrown away.

Start by tracing how terrain data from the server, uncompressed or packed meshes, tile requests, and upsampling are retained and used for picking. Investigate whether upsampling can rely on the retained mesh and whether limiting it to leaf nodes changes the imagery-versus-terrain loading behavior. Done means the redundant data retention and discarded upsampling work are addressed without losing picking or timely terrain display.

Written by the indexing model from the issue text.