Unified and fixed packing and unpacking computations.

Change-Id: I1ea2bcf28945a9f105152a6836f11bcb2859750c
Reviewed-on: https://swiftshader-review.googlesource.com/4715Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>