VertexRoutine code for Transform Feedback

VertexRoutine code for Transform Feedback

This cl adds the code that actually performs the copy of the
vertex shader outputs into transform feedback buffers. It
also contains a fix for symmetricNormalizedDepth, which must
be computed after the information was copied into the
transform feedback buffers, when transform feedback is active.

Change-Id: I418f94a15b9425bba0905c840f8cf4828233d0fb
Reviewed-on: https://swiftshader-review.googlesource.com/5172
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>