Proper validation of varyings used for transform feedback was added.
Simple types, as well as arrays and array elements are allowed.

Change-Id: I83ceb5eb19bf5691264ab0e142c3cc6a7debda4c
Reviewed-on: https://swiftshader-review.googlesource.com/5058Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>