Add compatibility for 21w39a and 21w43a snapshots

[26]

This pull request adds basic compatibility for 21w39a and 21w43a. In both these snapshots, changes were made to the way chunks are stored on disk.

It does not yet implement paletted biome's and I have not fully tested everything yet. However, a good-looking world seems to come out when I try to render a world created in 21w44a.

This pull request should fix #1982.

[26]

I have tested this on a much larger world. The result can be seen on https://poldercraft.nl/renders/world_141121/. I think this is ready now.

[Hackaloken]

I just started up a vanilla 1.18 server; this fixed map generation for me 👍

[spacegriffin]

Does this also fix rendering for the new generation under y=0?

[F-Node-Karlsruhe]

Urgently needed fix for 1.18 release

[bagobones]

Would appreciate this for minimum support. Our player group is really excited about 1.18 since so much has changed.

[BenKennish]

Can this be accepted please so that I can run a render for my 1.18 world?

[BenKennish]

OK so the players on my server are getting impatient to see the rendered map for the new 1.18 world. It's been a while since I did anything more than the basics with git so can someone advise me on how to get this code working on my server but in such a way that I can still upgrade to the latest official version of Overviewer when it's released (possibly with this PR or a variation of it)?

[elboletaire]

OK so the players on my server are getting impatient to see the rendered map for the new 1.18 world. It's been a while since I did anything more than the basics with git so can someone advise me on how to get this code working on my server but in such a way that I can still upgrade to the latest official version of Overviewer when it's released (possibly with this PR or a variation of it)?

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally