Gtk2::Gdk::Display->open and undef

[Kevin Ryde <user42 zip com au>]

I think gdk_display_open accepts NULL, to mean use the DISPLAY
environment variable (the same as XOpenDisplay), though the docs don't
seem to say so explicitly.

--- GdkDisplay.xs       30 Dec 2007 10:39:23 +1100      1.18
+++ GdkDisplay.xs       13 Apr 2008 16:27:02 +1000      
@@ -32,7 +32,7 @@
 
 ##  GdkDisplay *gdk_display_open (const gchar *display_name) 
 GdkDisplay_ornull *
-gdk_display_open (class, const gchar * display_name)
+gdk_display_open (class, const gchar_ornull * display_name)
     C_ARGS:
        display_name