Fixed the native font object so that it can find the X11 display

on Mac OS X even when the application is run outside an xterm.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402431

dlib/gui_widgets/nativefont.h

@@ -335,6 +335,15 @@ namespace nativefont
                     setlocale(LC_CTYPE, "");
                     if (d == NULL){
                         d = XOpenDisplay(NULL);
+                        if (d == 0)
+                        {
+                            d = XOpenDisplay(":0.0");
+                            if (d == 0)
+                            {
+                                throw dlib::gui_error("Unable to connect to the X display.");
+                            }
+                        }
+
                         cmap = DefaultColormap(d, DefaultScreen(d));
                     }
                     char fontset[256];