Terminal: Fix handling big chunks of OSC data

ShellIntegration previously ignored the initial and final flags of onOsc.

Change-Id: Ifa9594fba654cf543c117835fc182d4642dc70e4
Reviewed-by: Cristian Adam <cristian.adam@qt.io>

src/plugins/terminal/shellintegration.h

@@ -34,6 +34,7 @@ signals:
 
 private:
     QTemporaryDir m_tempDir;
+    QByteArray m_oscBuffer;
 };
 
 } // namespace Terminal