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/5172Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>