Textured Inventory Blocks

Make the inventory blocks match the blocks in the real world, textured and all!

Plus, the inventory blocks are now generated with the same code as the actual chunk blocks, which should make it easier to keep them in sync in the future.

I'm not totally happy with the code here, in particular I don't really like how much code items.js needs in order to mesh a single block (Like, it needs to know that we use uv and position attributes for indexed verticies). Ideally we would consolidate this knowledge in one place to make it easier to change in the future when we add lighting and such, but I'm not sure where/how. Suggestions welcome.