Loading of custom texture packs

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Given that this shouldn't be too complicated, seems like it could be a nice feature

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Well, Deepslate works off an atlas and I'm pretty sure making an atlas is NP-complete (rectangle packing) and I have no idea how Minecraft does it or where Deepslate gets that atlas. The JSON stuff is the easy part since it's just all the assets joined together (I'm not sure why they do atlas + joined JSON though, and as another note whether it follows copyright). Simplest I can think of is extracting the atlas from in-game (with a mod, perhaps) with resource packs loaded.

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Apparently Deepslate can load resource packs but I'm almost certain that it only works for 16px only ones without animated textures (where it just turns into tiling squares in a rectangle)... I guess worth implementing if it's simple enough.