Was an implicit narrowing from size_t (64-bit) to unsigned int (32-bit).

Change-Id: I9726d70370d97da1bae2652bbce9db929179a292
Reviewed-on: https://swiftshader-review.googlesource.com/13469Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>