The use of this extension is required to enable the MoltenVK physical
device as of Vulkan SDK 1.3.216.0. This is because MoltenVK is still
very (very) slightly non-conformant.
This updates to a newer version of glad2 and switches to the header-only
variant.
This also (finally) switches to the newer glad2 loader signature that
allows us to pass in glfwGetInstanceProcAddress directly.