pixbuf-demo: don't lock the window size

What's the use of an animation demo, if you can interactively
resize it and show off how cool is your window system handling
updates?

https://bugzilla.gnome.org/show_bug.cgi?id=725172

demos/pixbuf-demo/pixbuf-demo.c

@@ -195,7 +195,6 @@ main (int argc, char **argv)
 	window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
 
 	gtk_widget_set_size_request (window, back_width, back_height);
-        gtk_window_set_resizable (GTK_WINDOW (window), FALSE);
 
 	g_signal_connect (window, "destroy",
 			  G_CALLBACK (destroy_cb), NULL);