Fix signed/unsigned comparison warnings.

Bug 15387371

Change-Id: Id4c9b54c5c0b4115479b6710c4d8c91d34e5c002
Reviewed-on: https://swiftshader-review.googlesource.com/4494
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
24 files changed