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.
this produces a major performance improvement for large render distances, and reduces the impact of lighting calculation to zero on servers which have random blockupdates turned off.
it's useful to have an immutable stub around for the sake of feeding back dummy read values, but for write values it has to barf instead of being quiet.
There's still some issues with LightArray which I don't currently have a solution for, but I'm thinking about separating light storage from chunks anyway.
recalculateHeightMapColumn is stateless, so it can't make any assumptions about which subchunks to check for blocks. However, in most the average case (6 allocated subchunks), this causes 2500+ useless SubChunk->getHighestBlockAt() calls (10 per column). Since we're calculating in bulk, we can figure out which subchunks are empty one time and ignore them for all 256 columns.
In the average case, this produced a 50-60% performance improvement for heightmap calculation (~1.1 ms -> 0.5 ms).
In extreme cases where the height is extremely varied, this produces no observable performance benefit, but for most cases with flattish terrain, it's an improvement.
It can likely be further improved, but further performance improvements are outside the scope of this commit and will likely result in more complexity increases.
this was degraded whenever it was I decided to make chunks always be allocated. This commit uses a fast path for light filling in subchunks which are completely clear of the heightmap, which returns the performance back to its old fast levels.
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.
this is more efficient (less data copied for ITC), fixes#2873, and also fixes terrain changes during task run getting overwritten.
This still leaves the problem that the light information provided may be out of date by the time the task completes, but this is nonetheless a step forward.
