!UPSTREAM: Test core GS inputs on GLSL 1.50

TODO:
- Make this work.
!UPSTREAM: WIP gs tests

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

TODO:
- Add core outputs (gl_ClipDistance, gl_PointSize, and gl_PrimitiveID already done)

fixup! !UPSTREAM: gs: Test all core inputs except gl_PrimitiveIDIn.