Check that transform feedback size/type comes from gs, not vs.

When the current program has both a geometry shader and a vertex
shader, transform feedback needs to link with the geometry shader.
This test verifies that the type and size of the varyings captured by
transform feedback match their declarations in the geometry shader,
even if there are vertex shader outputs with the same names and
different types.

The test exercises one built-in shader output (gl_ClipDistance) and
one user-defined shader output.