Wayland: Use tags to verify proxy ownership

This is in preparation for adding support for libdecor, which creates
its own proxies on our display.  It will likely also be helpful to some
people using native access on Wayland.

This is partly based on the implementation of libdecor support in
PR #1693 by @ christianrauch.

src/wl_monitor.c

@@ -191,6 +191,7 @@ void _glfwAddOutputWayland(uint32_t name, uint32_t version)
     monitor->wl.output = output;
     monitor->wl.name = name;
 
+    wl_proxy_set_tag((struct wl_proxy*) output, &_glfw.wl.tag);
     wl_output_add_listener(output, &outputListener, monitor);
 }