Clang 9 escalates a warning about an unsigned integer being compared
against a signed integer into an error.

Fixed by making an integer literal be unsigned.

Bug: b/152777669
Change-Id: I38c61ac0837f756c73b8664f7774719a94353fac
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/51530Tested-by: Sean Risser <srisser@google.com>
Commit-Queue: Sean Risser <srisser@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>