Fix fatal X11 errors when both external display and command arguments are provided to QApplication

This resolves Bug 1456

src/kernel/qapplication_x11.cpp

@@ -1803,6 +1803,13 @@ void qt_init_internal( int *argcptr, char **argv,
 	    }
 	}
 #endif
+	if ( display ) {
+		// Display connection already opened by another application
+
+		appForeignDpy = TRUE;
+		appDpy  = display;
+	}
+	else {
 		// Connect to X server
 	
 		if( qt_is_gui_used ) {
@@ -1817,6 +1824,7 @@ void qt_init_internal( int *argcptr, char **argv,
 			XSynchronize( appDpy, TRUE );
 		}
 	}
+    }
     // Common code, regardless of whether display is foreign.
 
     // Get X parameters