glsl: Disable transform feedback of varying structs.

glsl: Disable transform feedback of varying structs.

It is not clear from the GLSL ES 3.00 spec how transform feedback is
supposed to apply to varying structs:

- There is no specification for how the structure is to be packed when
  it is recorded into the transform feedback buffer.

- There is no reasonable value for GetTransformFeedbackVarying to
  return as the "type" of the variable.

We currently have a Khronos bug requesting clarification on how this
feature is supposed to work
(https://cvs.khronos.org/bugzilla/show_bug.cgi?id=9856).

This patch just disables transform feedback of varying structs for
now; we can implement the proper behaviour once we find out from
Khronos what it is.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>