Use the given YV12 texture stride instead of computing it from width.

Bug 25690690

Change-Id: If25a8868effb1e55908c4041555e553b52ee789a
Reviewed-on: https://swiftshader-review.googlesource.com/4292
Reviewed-by: Keun Soo Yim <yim@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>