window: Remove display_flush_cairo_device

Rather add a flush_surface utility that
flushed the cairo_device thats associated to a surface.
diff --git a/clients/cairo-util.h b/clients/cairo-util.h
index 257489a..e490b9f 100644
--- a/clients/cairo-util.h
+++ b/clients/cairo-util.h
@@ -24,6 +24,9 @@
 #define _CAIRO_UTIL_H
 
 void
+surface_flush_device(cairo_surface_t *surface);
+
+void
 blur_surface(cairo_surface_t *surface, int margin);
 
 void