preserve $DISPLAY

direct/src/plugin/p3dSession.cxx

@@ -761,6 +761,9 @@ start_p3dpython(P3DInstance *inst) {
     "TMP", "TEMP", "HOME", "USER", 
 #ifdef _WIN32
     "SYSTEMROOT", "USERPROFILE", "COMSPEC", "PANDA_ROOT",
+#endif
+#ifdef HAVE_X11
+    "DISPLAY",
 #endif
     NULL
   };