xwayland: Add an output name for fullscreen

Add a output name to the xwl_screen.

This is preparation work for fullscreen rootful on a specific output,
no functional change.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Acked-by: Michel Dänzer <mdaenzer@redhat.com>

hw/xwayland/xwayland-screen.h

@@ -145,6 +145,7 @@ struct xwl_screen {
     int libdecor_fd;
     struct libdecor *libdecor_context;
 #endif
+    const char *output_name;
 };
 
 /* Apps which use randr/vidmode to change the mode when going fullscreen,