Varying variables can be structure in OpenGL ES3.
This cl adds support for structure varyings.

Change-Id: I4d1d80c6afed0a86a23b0a467d4764a4e08f133d
Reviewed-on: https://swiftshader-review.googlesource.com/13529Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>