Make sure `uv`s for the texture array-based fragment shader are not interpolated

[bzm3r]

Currently, we rely on casting uv.z as i32. Does this have any weird edge cases? Better thing would be to add in a flag on the uv that specifies it should not be interpolated. See here for an example of how to do this: https://github.com/bzm3r/doug_renderers/blob/428a400d85ba989e5f78d5d7b4eb9686334bc12c/src/shaders/vpull.wgsl#L43