!UPSTREAM: GL 3.1: test that core features are properly excluded from old GLSL versions.

TODO:
- Do I need a better commit message?
- Test on nVidia
!UPSTREAM: gs: Test all core inputs except gl_PrimitiveIDIn.

gl_PrimitiveIDIn will need more aggressive testing, so that will be
addressed in a separate patch.

TODO:
- Add core outputs