Provide default constructors for each type, and initialize the swizzle
parent in a less bug prone manner.

Change-Id: Ia7f406a66274c7b1737cbe82db733e2e3ab65175
Reviewed-on: https://swiftshader-review.googlesource.com/8453Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>