pan/layout: add const to immutable local vars in pan_image_layout_init

Putting compiler optimization aside, this at least improves readability
by making it clean which vars are mutable and which are immutable.

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35299>
1 file changed