Provide visual feedback as to whether drop is supported

Change the cursor in response to target events.
diff --git a/clients/window.c b/clients/window.c
index 24f1a70..c6c36e9 100644
--- a/clients/window.c
+++ b/clients/window.c
@@ -407,6 +407,12 @@
 	cairo_device_flush (window->display->device);
 }
 
+void
+display_flush_cairo_device(struct display *display)
+{
+	cairo_device_flush (display->device);
+}
+
 static void
 window_draw_fullscreen(struct window *window)
 {