Previously the derivatives in y were computed using the difference
between the second and fourth pixel in the quad. For consistency with
the x derivative, use the first and third pixel instead. Also, some
shuffling can be eliminated by having the x derivative in the y
component, and the y derivative in the z component.

Change-Id: I985dcd3c5e2c47c10caf020cf5cb79587b3b3aab
Reviewed-on: https://swiftshader-review.googlesource.com/14168Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>