TRANSFORM_FEEDBACK_VARYING corresponds to the set of output variables
in the last non-fragment stage of program that would be captured when
transform feedback is active. The resources enumerated by this query
are listed as specified by the most recent call to
TransformFeedbackVaryings before the last call to LinkProgram.
This mainly collects these resources for query.

BUG=angleproject:1920
TEST=angle_end2end_tests:ProgramInterfaceTest*
     dEQP-GLES31.functional.program_interface_query.transform_feedback_varying.*

Change-Id: I0655b12c6d82cef1b44d4ca57ea55bb60d1f78fb
Reviewed-on: https://chromium-review.googlesource.com/770450
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>