Use this function to get the address of the ```vkGetInstanceProcAddr``` function.
void *SDLCALL SDL_Vulkan_GetVkGetInstanceProcAddr()
Returns a pointer to the vkGetInstanceProcAddr which can be used to load Vulkan function pointers at runtime.
This should be called after either calling SDL_Vulkan_LoadLibrary or creating a window with the SDL_WINDOW_VULKAN flag.
This function is available in SDL 2.0.8