wgpu backend: Remove unneeded cloned() when creating texture arrays

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Related to #107

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Also related to #7

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Lol it didn't work

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After struggling a bit, it turns out there's two ways to write this without cloned

let slice: Vec<u8> = layers.iter().map(|i| i.iter()).flatten().map(|i| *i).collect();

let slice: Vec<u8> = layers.iter().map(|i| *i).flatten().map(|i| *i).collect();

(playground link)

But in the end, it probably doesn't matter because u8 is small and Copy (cloning it isn't "bad"). So I'm closing it.