Release host memory for distant voxel chunks

[Ralith]

Although we already carefully manage video memory, we currently store the original voxel data in host memory for the duration of the process. To limit memory use when exploring large areas, we should free distant chunks, and regenerate them in the rare event that they're needed again.

A straightforward approach would be storing actual voxel data in a new LRU table, indexed into by the graph.