This cl doesn't entirely enable integer texture sampling, but
it adds the required cases in SamplerCore. To fully enable
these types, only a few small selection functions will be
required so that the proper types use the proper sampling code.

Change-Id: Ie101d782dd9b43c96d8e0198c49fe9ec6855b007
Reviewed-on: https://swiftshader-review.googlesource.com/4150Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>