Previously we assumed the source stride (in pixels) to be the width
rounded up to the next multiple of 2. This is currently correct but
may not hold in the future.

Change-Id: I23a79ace66f720398a120b70081d731c2cf1b4c0
Reviewed-on: https://swiftshader-review.googlesource.com/4481Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>