Implemented the API functions to set and get
the varyings used for transform feedback.

Change-Id: I0d6451cfbd4a4b1b96dd9c064bb9b310b46764c4
Reviewed-on: https://swiftshader-review.googlesource.com/3462Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>