vk: Add an initial implementation of the actual Khronos WSI extension

vk: Add an initial implementation of the actual Khronos WSI extension

Unfortunately, this is a very large commit and removes the old LunarG WSI
extension.  This is because there are a couple of entrypoints that have the
same name between the two extensions so implementing them both is
impractiacl.

Support is still incomplete, but this is enough to get vkcube up and going
again.