If changed && append, we shouldn't be resetting the internal offset back
to zero.  This fixes issues w/ sequences like:

   glBeginTransformFeedback()
   glDraw()
   glPauseTransformFeedback()
   glDraw()
   glResumeTransformFeedback()
   glDraw()
   glEndTransformFeedback()

Fixes dEQP-GLES3.functional.transform_feedback.array.separate.points.lowp_vec3
and related tests.

Signed-off-by: Rob Clark <robclark@freedesktop.org>