Optimize Distant Terrain memory consumption
Basically, Distant Terrain causes no issues with clean game, but with Tamriel Rebuilt is consumes near 2GB of memory itself, also there are freezes during turning. On machines with low amount of available memory it can cause "out of memory" errors, especially on x86.
It seems we load data for the whole game world.
- Re-use data for "default" underwater nodes.
- Unload data, if we do not render this cell.