Fix indexing of samplers in structures.

Change-Id: I0bde678865902d549d4260bf8f6c1a368a46f4d4
Reviewed-on: https://swiftshader-review.googlesource.com/5191
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>