Eliminate the half-pixel NDC shift.

Bug 22123818

Change-Id: If9bf78d6b44ccd0662676c8896837cca8829efc8
Reviewed-on: https://swiftshader-review.googlesource.com/3624
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>