Whenever a cube map was being used, these asserts would fire as soon
as any border computation would happen. Fixed the asserts to allow
cube maps to work properly.

Change-Id: Iedc2661e63db37e5b4e77e08f97ce044e9f88837
Reviewed-on: https://swiftshader-review.googlesource.com/19468Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>