Vulkan: vkEnumerateInstanceVersion support
Glad could export a function to enumerate the Vulkan Loader support if we don't use the glad's loader part. Something like:

```
PFN_vkEnumerateInstanceVersion my_EnumerateInstanceVersion =
    (PFN_vkEnumerateInstanceVersion)vkGetInstanceProcAddr(
        VK_NULL_HANDLE, "vkEnumerateInstanceVersion");
if (nullptr != my_EnumerateInstanceVersion) {
    // Command is present and defined.
} else {
    // Command is not present, treat as Vulkan 1.0 only
}
```