Add support for compatibility contexts
This MR fixes all the piglits related to compatibility contexts.
It lowers the use of TGSI_SEMANTIC_CLIPVERTEX and handles TGSI_SEMANTIC_COLOR in all shaders. In addition some patches fix issues that deal with problems that may be introduced by the upcoming nir-to-tgsi compiler code path.
Dying Light suposedly needs a compatibility profile, but it still renders crap, so maybe there are things still missing.
Mesa patch: mesa/mesa!12542 (merged)