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

This resolves Bug 1456
(cherry picked from commit 7abfc2a181)

src/kernel/qapplication_x11.cpp

@@ -1790,18 +1790,26 @@ void qt_init_internal( int *argcptr, char **argv,
 	    }
 	}
 #endif
-	// Connect to X server
-
-	if( qt_is_gui_used ) {
-	    if ( ( appDpy = XOpenDisplay(appDpyName) ) == 0 ) {
-		qWarning( "%s: cannot connect to X server %s", appName,
-			  XDisplayName(appDpyName) );
-		qApp = 0;
-		exit( 1 );
-	    }
+	if ( display ) {
+		// Display connection already opened by another application
 
-	    if ( appSync )				// if "-sync" argument
-		XSynchronize( appDpy, TRUE );
+		appForeignDpy = TRUE;
+		appDpy  = display;
+	}
+	else {
+		// Connect to X server
+	
+		if( qt_is_gui_used ) {
+		if ( ( appDpy = XOpenDisplay(appDpyName) ) == 0 ) {
+			qWarning( "%s: cannot connect to X server %s", appName,
+				XDisplayName(appDpyName) );
+			qApp = 0;
+			exit( 1 );
+		}
+	
+		if ( appSync )				// if "-sync" argument
+			XSynchronize( appDpy, TRUE );
+		}
 	}
     }
     // Common code, regardless of whether display is foreign.