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This still leaves the problem that the light information provided may be out of date by the time the task completes, but this is nonetheless a step forward.
