vk/wsi: Move to a clallback system for the entire WSI implementation

vk/wsi: Move to a clallback system for the entire WSI implementation

We do this for two reasons: First, because it allows us to simplify WSI and
compiling in/out support for a particular platform is as simple as calling
or not calling the platform-specific init function.  Second, the
implementation gives us a place for a given chunk of the WSI to stash
stuff in the instance.