vendor:vulkan Add support for xlib and xcb

[flga]

We need forward declarations for the WSI types, but xlib doesn't use any prefixes.
To make it clear and consistent with the other WSI types, I've added the Xlib prefix:

Display => XlibDisplay
Window => XlibWindow
VisualID => XlibVisualID

However, one could argue keeping the names consistent with xlib would be better, so let me know your thoughts on that.

[laytan]

Could you not use vendor:x11/xlib for the types?

[flga]

Done.

I thought that importing x11/xlib would imply xlib gets linked in, but anecdotally that seems to not be the case, the dependency gets elided if no procedures are called.

Can this be relied on will things like reflect get in the way of this "optimization"?

xlib has build constraints that have to be either matched or lifted. I chose the former for no other reason than trying to keep it consistent with win32.
This does mean that they need to be kept in sync but I suspect that's not a big deal.

PS: there's a 3rd option here, which would be to match vulkan and separate out WSI stuff into independent sub packages, but that's both breaking and more intrusive

[laytan]

Yeah foreign dependencies don't get linked if none of their procs are called.