Use safer typed enums.

Bug 19331817

Change-Id: I75787c6a9df3bca539dcaf3a1d3f0e49994c73b4
Reviewed-on: https://swiftshader-review.googlesource.com/2390
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>