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.
This removes all dependencies from the GLFW test programs on the Vulkan
SDK.
It also removes support for linking the GLFW shared library (dynamic
library, DLL) against the Vulkan loader static library.