Mir Continues Cleaning Up Their OpenGL Code, To Support Vulkan In Future

Since this summer we've known that Canonical developers have been looking at Vulkan in regards to supporting this forthcoming graphics API by Unity 8 and Mir. Since then we've seen work done in Mir to support renderers other than OpenGL with this Ubuntu display server. As another sign of working towards Vulkan, more of Mir's OpenGL code continues to be re-factored...