client: Fix shell unstable version check

Use a static assert to catch mismatch between implementation and
interface version.  Fix window.c to not use XDG_SHELL_VERSION_CURRENT,
which will fail to catch version mismatches.  The implementation version
must updated manually when the implementation is updated to use the new
interface.

desktop-shell/shell.c

@@ -3635,7 +3635,7 @@ xdg_shell_unversioned_dispatch(const void *implementation,
 		return 0;
 	}
 
-#define XDG_SERVER_VERSION 1
+#define XDG_SERVER_VERSION 2
 
 	static_assert(XDG_SERVER_VERSION == XDG_SHELL_VERSION_CURRENT,
 		      "shell implementation doesn't match protocol version");