Zink Enables OpenGL Threading For "Huge Perf Gains"

Recently the open-source AMD OpenGL driver "RadeonSI" enabled OpenGL threading by default for the "glthread" option that has long been opt-in on a per-game/app basis. Along with that has been a number of glthread-related improvements to this code that punts executing OpenGL calls to a separate CPU thread. The Zink OpenGL-on-Vulkan driver has now unconditionally enabled OpenGL threading too...