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.c b/clients/cairo-util.c
index 7bcfe41..184f58e 100644
--- a/clients/cairo-util.c
+++ b/clients/cairo-util.c
@@ -31,6 +31,16 @@
 #define ARRAY_LENGTH(a) (sizeof (a) / sizeof (a)[0])
 
 void
+surface_flush_device(cairo_surface_t *surface)
+{
+	cairo_device_t *device;
+
+	device = cairo_surface_get_device(surface);
+	if (device)
+		cairo_device_flush(device);
+}
+
+void
 blur_surface(cairo_surface_t *surface, int margin)
 {
 	int32_t width, height, stride, x, y, z, w;