X11 and XCB WSI on macOS

[DDoS]

**What enhancement are you suggesting for the Vulkan Loader? Please describe in detail. ** The Lavapipe software driver from Mesa can be built on macOS and supports the X11 and XCB WSI. The loader doesn't build with X support on Darwin, but by editing only the CMake options to allow it, it does build and run.

This is a software renderer, so 5FPS is expected. It's about the same frame rate as when using the Metal WSI.

Only issue I found so far is that the validation layer wrongly complains "invalid" VkSurfaceKHR and the loader crashes on a null surface. But if the validation layer is disabled it works.

Is this specific to a single platform? macOS

Additional context I'll admit this is a pretty niche feature, and Lavapipe has supported the Metal WSI for a few months now, but it's cool to have for cross-platform development.