Added initialization for swizzle related members of Sampler

Change-Id: I81f7aad5274696390e73c32ff1b424687e9f5943
Reviewed-on: https://swiftshader-review.googlesource.com/4111Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>