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

[dlight]

Related to #107

[dlight]

Also related to #7

[dlight]

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.