Adjusting the cube-dimensions to account for layers and faces does
nothing to change the number of elements, it just shifts the number
between the two.
This means we can simplify things a bit, and avoid a questionable assert
in the process.
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28909>
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