The addressing is offset by 1 to account for the border. Note that this
could be avoided by locking at (0, 0) instead of (-1, -1) instead, but
then negative address offsets have to be allowed and this complicates
the subsequent calculations and only unsigned extension from 32-bit to
64-bit is typically for free during memory accesses.

Change-Id: I5fb843401d440a9d77d141782124a9c260765830
Reviewed-on: https://swiftshader-review.googlesource.com/13289Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>