This test verifies that transform feedback can be applied to the
following built-in vertex shader outputs:

- gl_FrontColor
- gl_BackColor
- gl_FrontSecondaryColor
- gl_BackSecondaryColor
- gl_TexCoord
- gl_FogFragCoord
- gl_Position
- gl_PointSize
- gl_ClipVertex
- gl_ClipDistance

I've verified that all of these tests pass on the nVidia proprietary
Linux driver except for gl_ClipVertex (which is unsurprising, as
gl_ClipVertex has several known bugs on this driver).