The unpack row length was supposed to replace the width, not the
pitch, of the input image, which explains why related tests were
failing.

Change-Id: I7bd46e057bae4113b31d072fd6748d9dd6043feb
Reviewed-on: https://swiftshader-review.googlesource.com/4080Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>