Fix supporting samplers in structs passed down as function arguments.

Bug 19354873

Change-Id: Ic21a36eefcdef4d9fcce1952fa15425da8858200
Reviewed-on: https://swiftshader-review.googlesource.com/2260
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>