Support per surface frame events

This fixes tearing with multi head.
diff --git a/clients/window.h b/clients/window.h
index 7073f4a..c5285f7 100644
--- a/clients/window.h
+++ b/clients/window.h
@@ -157,6 +157,9 @@
 cairo_surface_t *
 window_get_surface(struct window *window);
 
+struct wl_surface *
+window_get_wl_surface(struct window *window);
+
 void
 window_flush(struct window *window);