Add tests for propagation of invariant qualifier
Relevant merge request into Mesa: mesa!84 (closed)
The tests exercise two issues:
- Mesa's GLSL bug where 'invariant' qualifier propagates on one definition of the global variable defined in several shaders of one stage causing linking error due to qualifier mismatch.
- Mesa's GLSL bug where 'invariant' qualifier propagated on uniforms causing a linker failure due to mismatch of uniforms declaration between stages.
The tests where moved from mailing lists.