Make IsTransformFeedback() return false before BindTransformFeedback()

Make IsTransformFeedback() return false before BindTransformFeedback()

The GL 4.3 an ES 3.0 specs say

   A transform feedback object is created by binding a name returned by
   GenTransformFeedbacks with the command
      void BindTransformFeedback( enum target, uint id );

Fixes arb_transform_feedback2-istransformfeedback and part of
es3conform's CoverageES30.test.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>