Nearly all of the functionality of the EXT_transform_feedback
extension is available in core OpenGL as of version 3.0 (only one
function was not promoted to OpenGL 3.0, the BindBufferOffsetEXT
function).  Therefore, it's possible for an implementation to support
transform feedback via core OpenGL 3.0 features, without advertising
support for the EXT_transform_feedback extension.  One such
implementation, surprisingly, is the nVidia proprietary Linux driver I
use as my reference platform.

To allow the transform feedback tests to run on these implementations,
I've created a function piglit_require_transform_feedback(), and the
following function pointers:

- piglit_BeginTransformFeedback
- piglit_BindBufferBase
- piglit_BindBufferRange
- piglit_EndTransformFeedback
- piglit_GetBooleani_v
- piglit_GetIntegeri_v
- piglit_GetTransformFeedbackVarying
- piglit_TransformFeedbackVaryings

piglit_require_transform_feedback() checks whether transform feedback
is provided via EXT_transform_feedback or via core GL 3.0, and sets up
the function pointers appropriately.  If transform feedback isn't
provided through either mechanism, then it causes the test to be
skipped.

This is similar to what we do with piglit_require_GLSL() (see
tests/util/piglit-shader-gl.c).

I've updated all of the EXT_transform_feedback tests to use these
function pointers.  This makes it possible to test transform feedback
on an implementation that supports GL 3.0 but not
EXT_transform_feedback.

Acked-by: Marek Olšák <maraeo@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>