From the ARB_separate_shader_objects spec v.25:

  " To use any built-in input or output in the gl_PerVertex and
    gl_PerFragment blocks in separable program objects, shader code
    must redeclare those blocks prior to use.  A separable program
    will fail to link if:

      * it contains multiple shaders of a single type with different
        redeclarations of these built-in input and output blocks; or

      * any shader uses a built-in block member not found in the
        redeclaration of that block."

Additionally, added the "SEPARABLE PROGRAM ENABLED" directive to the
shader_runner

Signed-off-by: Andres Gomez <agomez@igalia.com>
Acked-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>