Implement predefined texture functions.

Bug 21278131

Change-Id: I16f27aeaf530f7672e4f60ea379ec82450cc5243
Reviewed-on: https://swiftshader-review.googlesource.com/3230
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>