Properly destroying wl_registry

wl_registry does not have a destructor request. While clients can destroy the client-side proxy, the server-side resource will remain until the Wayland connection closes. Originally this was thought to be not a problem, because there is no reason for a client to create more than one wl_registry instance in a connection's life time. Except there are reasons due to existing API design. E.g. Vulkan has no place to cache connection-wide data, and even if it had, I hear the locking requirements do not allow it to work. I believe Vulkan is far from the only API having this problem as Wayland-related component libraries tend to create their own registries, although maybe not as pathological as Vulkan.

We cannot easily version wl_registry to add new messages, because it is created directly from wl_display which has a fixed version. Ideas like #106 have floated around. This is another idea:

Define a protocol extension, advertised through wl_registry, whose global interface has a request destroy_registry(wl_registry).

This would need an extension definition XML file (wayland-fixes.xml in libwayland? or wayland-protocols? or just addition in wayland.xml?), and a server-side implementation in libwayland-server. The extension could be automatically added when a server struct wl_display is created.

Client-side implementation would have to be manual, or rely on two new libwayland-client entry points:

• A blocking function that establishes any wayland-fixes extensions available and stores them with the client struct wl_display. I believe this is necessary to avoid making existing functions unexpectedly blocking.
• A function to destroy wl_registry that uses the extension if it was available, or uses the old way otherwise. This is necessary, because the existing wl_registry_destroy() is a static inline generated function calling wl_proxy_destroy().

While these two changes need to be done in all client code, I think they are worth it. It avoids unexpected changes in function behavior. The extension establishing function could be used for other fixes as well, if the need arises.