Apply xoffset, yoffset, and zoffset at image buffer lock so we don't
have to pass them down to the row loading functions as parameters.

Change-Id: I055549c80b1b207df4c49f6f59ecfb8286736040
Reviewed-on: https://swiftshader-review.googlesource.com/17068Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>