this is slightly slower, but saves a significant amount of memory (~80 KB per chunk).
Since ext-chunkutils2 doesn't do copy-on-write trickery like the PHP impl did, we need this to get the memory advantages back.
I think we should probably get rid of this considering the potential for inconsistencies within a chunk, but not retaining this is a bug nonetheless, even though it doesn't have any effect in PM itself since we always use BlockLegacyIds << 4 as the empty block ID.
so, this is only really aiding (ab)use cases which weren't intended anyway ...
this better describes the purpose, which is to identify air.
though, it might make more sense to make air just always have zero as air's runtime ID, since this parameter is apparently making plugin devs think that this is suitable to fill a chunk with a specific block ...
copy-on-write and zero-layer SubChunk objects are much easier to manage and have less cognitive overhead.
obviously, the goal is to get rid of EmptySubChunk completely, but right now it does still serve a purpose (filling in dummy values for reading out-of-bounds on chunks), and that's a problem that takes a little more work to fix.
