Wayland: Use Wayland to wait for libdecor to init

Much of libdecor is initialized only after certain events have been
received from the compositor and some parts of libdecor 0.1 are unsafe
to use until this delayed initialization has completed.

Since libdecor does not provide an API to query if or be notified when
this has happened, GLFW processed events until its newly created
libdecor frame had created its XDG shell objects.

This commit switches to using a generic Wayland sync point created just
after libdecor (and presumably its plugin) has set up its delayed
initialization, instead of relying on the more specific implementation
detail mentioned above.

It also makes this wait mandatory before the first libdecor frame is
created instead of a pre-condition for certain libdecor frame calls,
hopefully removing even more dependence on implementation details.

src/wl_platform.h

@@ -550,6 +550,8 @@ typedef struct _GLFWlibraryWayland
     struct {
         void*                   handle;
         struct libdecor*        context;
+        struct wl_callback*     callback;
+        GLFWbool                ready;
         PFN_libdecor_new        libdecor_new_;
         PFN_libdecor_unref      libdecor_unref_;
         PFN_libdecor_get_fd     libdecor_get_fd_;