a non-generated chunk is now always represented by NULL. This forces the case of ungenerated chunks to be handled by all code, which is necessary because ungenerated chunks cannot be interacted with or modified in any meaningful way.
in cases like PopulationTask it makes more sense to store the coordinates separately where they can be stored more efficiently (once instead of 9 times)
In addition, PopulationTask shouldn't need to serialize an empty chunk just to copy coordinates.
I've made changes like this in other areas already in preparation for the day when chunks no longer contain their coordinates, so this brings us one step closer to that goal.
this allows injection of arbitrary ChunkManagers into a single Generator instance.
The objective here was to remove the requirement to cache a SimpleChunkManager instance in worker-local storage, because that requires that the chunks it stores be manually removed to avoid memory leaks. However, there are some other obstacles, primarily the worldHeight which is not retained anywhere else.
this restores SimpleChunkManager's behaviour to PM3, removing the need for GeneratorChunkManager (although I'm dubious whether SubChunkExplorer makes any sense in there any more now that we have morton in the mix).
non-populated chunks shouldn't be light-populated anyway, but in some cases they are (bug or plugin interference).
chunks which were already populated might get modified by adjacent chunk populations, which should invalidate their lighting because generation doesn't track which blocks were changed, so the whole chunk should be recalculated.
this new form allows skipping some useless checks during sky light calculation and also allows getting rid of the last hard dependency on core Block classes.
We're getting real close to native light now.
