Added get/set functions for binding
a transform feedback buffer.

Change-Id: I409841a35644ff8d5f99c0a6b4c3a43568676822
Reviewed-on: https://swiftshader-review.googlesource.com/2930Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>