window: Add API to check for subcompositor, use in subsurface example

This way we can fail with a nice error message.
diff --git a/clients/window.h b/clients/window.h
index a817734..1646d5a 100644
--- a/clients/window.h
+++ b/clients/window.h
@@ -78,6 +78,9 @@
 struct wl_display *
 display_get_display(struct display *display);
 
+int
+display_has_subcompositor(struct display *display);
+
 cairo_device_t *
 display_get_cairo_device(struct display *display);