compositor: Set WAYLAND_DISPLAY if we're given a --socket argument

This lets us run multiple compositors (nested or on different VTs).

https://bugs.freedesktop.org/show_bug.cgi?id=46629

src/compositor.c

@@ -3536,6 +3536,8 @@ int main(int argc, char *argv[])
 
 	config_file = config_file_path("weston.ini");
 	parse_config_file(config_file, cs, ARRAY_LENGTH(cs), shell);
+	if (socket_name)
+		setenv("WAYLAND_DISPLAY", socket_name, 1);
 
 	backend_init = load_module(backend, "backend_init", &backend_module);
 	if (!backend_init)