Enable loading and rendering of multiple regions

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See title.

Only question is do we just dump all blocks into one big structure, or make separate structures.

[Undecentions]

Did a quick patch since I needed to use this and things weren't rendering (I also noticed when rendering some ice farms some months ago and other things before that), had some trouble with negatives on region sizes for whatever reason they exist for (I suspect the original had trouble too?), and now I have an upside-down Dark's cannon.

Easily fixable though.

I'd say dump in one, unless there's an easy way to get deepslate to render multiple and have the grids not collide with everything possible.

[Undecentions]

By the way I'm so not used to forums... anyways the cannon is the right orientation now and here's the sketchy patch I did (under the other things)...
Also the ice farms fail due to the invisible block mesh being too big... not sure why that even exists or what it does, but I removed it and things rendered properly... hmm.

So far no problems (other than the usual no BE rendering).

function structureFromLitematic(litematic) {
  let width = height = depth = 0;
  let global_x = global_y = global_z = Infinity;

  for (const region of litematic.regions) {
    global_x = Math.min(global_x, region.x, region.x + region.width);
    global_y = Math.min(global_y, region.y, region.y + region.height);
    global_z = Math.min(global_z, region.z, region.z + region.depth);
    width = Math.max(width, region.x, region.x + region.width);
    height = Math.max(height, region.y, region.y + region.height);
    depth = Math.max(depth, region.z, region.z + region.depth);
  }

  const structure = new deepslate.Structure([width - global_x, height - global_y, depth - global_z]);

  // Add blocks from litematic NBT
  console.log("Building", litematic.regions.length, "regions")
  for (let region of litematic.regions) {
    console.log("Building region start.");
    // When lazy to do anything so just unpack
    const {x: origin_x, y: origin_y, z: origin_z, blocks, blockPalette} = region;
    let blockCount = 0; // What do we do with this?

    // Unless there's a way to do this easier
    const dx = region.width > 0 ? 1 : -1;
    const dy = region.length > 0 ? 1 : -1;
    const dz = region.depth > 0 ? 1 : -1;
    const start_x = Math.min(0, region.width);
    const start_y = Math.min(0, region.height);
    const start_z = Math.min(0, region.depth);

    // One reason js is annoying... the funny thing is that
    // this whole thing can be done easily with Python's
    // negative indicies for negative region sizes
    for (let x = start_x; x < Math.max(0, region.width); x++) {
      for (let y = start_y; y < Math.max(0, region.height); y++) {
        for (let z = start_z; z < Math.max(0, region.depth); z++) {
          const blockID = blocks[x - start_x][y - start_y][z - start_z];
          const position = [x + origin_x - global_x, y + origin_y - global_y, z + origin_z - global_z];
          if (blockID > 0) { // Skip air-blocks
            if(blockID < blockPalette.length) {
              const blockInfo = blockPalette[blockID];
              const blockName = blockInfo.Name;
              blockCount++;

              if (blockInfo.hasOwnProperty("Properties")) {
                structure.addBlock(position, blockName, blockInfo.Properties);
              } else {
                structure.addBlock(position, blockName);
              }
            } else {
              // Something obvious so we know when things go wrong
              // Why not throw some error or something
              structure.addBlock(position, "minecraft:stone");
            }
          }
        }
      }
    }
    console.log("Region done.", blockCount, "blocks created.");
  }
  console.log("Done!");

  return structure;
}

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By the way, only reason I'm not making a PR at this point is if there's a strong reason to use multiple structures.

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Demo in dev branch of fork (deployed at https://undecentions.github.io/litematic-viewer/)

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I should have checked here first - have added my own implementation to dev

TODO (maybe): enable selective rendering of different regions?

[Undecentions]

This is pretty similar to the layer rendering thing, currently (in my implementation) it adds each region separately, so it should be pretty simple to add a condition for each.
Also, just a side note, I recommend that you split some of the commits more so it's easier to read, although with the current scale, it's still manageable.

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Yes, I ended up just doing a bunch of refactoring along the way and didn't really think to do an intermediate commit

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Update: Deepslate fixed (misode/deepslate#20) their thing, so now we can use the unpkg link instead of having to embed the js file. I'm currently too lazy to mess with the new code though, so I'll just update my fork for now.

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Update: I'm no longer so sure that they fixed their thing, since that fix was really old so I probably tested that already, and the code's doing weird stuff again,

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As for the big chunk of code above, make sure to change according to https://stackoverflow.com/questions/16975350/assign-multiple-variables-to-the-same-value-in-javascript which I didn't know at the time.

[Undecentions]

(And the unused dx, dy, dz which I also somehow didn't notice at the time. Basically, run it through ESLint.)