We have only a few callers of unpack that do rects, so add a helper that iterates over y adding the strides. This saves us 36kb of generated code and means that adding cpu-specific variants for RGBA format unpack will be much simpler. Reviewed-by: Jesse Natalie <jenatali@microsoft.com> Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10014>