arb_separate_shader_objects: add gl_PerVertex tests
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